LibWeb: Handle currently ignored `WebIDL::ExceptionOr<T>`s

Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp

@@ -2677,14 +2677,14 @@ JS_DEFINE_NATIVE_FUNCTION(@prototype_class@::@attribute.setter_callback@)
             if (attribute.extended_attributes.contains("Reflect")) {
                 if (attribute.type->name() != "boolean") {
                     attribute_generator.append(R"~~~(
-    impl->set_attribute(HTML::AttributeNames::@attribute.reflect_name@, cpp_value);
+    MUST(impl->set_attribute(HTML::AttributeNames::@attribute.reflect_name@, cpp_value));
 )~~~");
                 } else {
                     attribute_generator.append(R"~~~(
     if (!cpp_value)
         impl->remove_attribute(HTML::AttributeNames::@attribute.reflect_name@);
     else
-        impl->set_attribute(HTML::AttributeNames::@attribute.reflect_name@, String::empty());
+        MUST(impl->set_attribute(HTML::AttributeNames::@attribute.reflect_name@, String::empty()));
 )~~~");
                 }
             } else {

Userland/Libraries/LibWeb/Bindings/AudioConstructor.cpp

@@ -45,7 +45,7 @@ JS::ThrowCompletionOr<JS::Object*> AudioConstructor::construct(FunctionObject&)
     auto audio = DOM::create_element(document, HTML::TagNames::audio, Namespace::HTML);
 
     // 3. Set an attribute value for audio using "preload" and "auto".
-    audio->set_attribute(HTML::AttributeNames::preload, "auto"sv);
+    MUST(audio->set_attribute(HTML::AttributeNames::preload, "auto"sv));
 
     auto src_value = vm.argument(0);
 
@@ -53,7 +53,7 @@ JS::ThrowCompletionOr<JS::Object*> AudioConstructor::construct(FunctionObject&)
     //    (This will cause the user agent to invoke the object's resource selection algorithm before returning.)
     if (!src_value.is_undefined()) {
         auto src = TRY(src_value.to_string(vm));
-        audio->set_attribute(HTML::AttributeNames::src, move(src));
+        MUST(audio->set_attribute(HTML::AttributeNames::src, move(src)));
     }
 
     // 5. Return audio.

Userland/Libraries/LibWeb/Bindings/ImageConstructor.cpp

@@ -47,13 +47,13 @@ JS::ThrowCompletionOr<JS::Object*> ImageConstructor::construct(FunctionObject&)
     // 3. If width is given, then set an attribute value for img using "width" and width.
     if (vm.argument_count() > 0) {
         u32 width = TRY(vm.argument(0).to_u32(vm));
-        image_element->set_attribute(HTML::AttributeNames::width, String::formatted("{}", width));
+        MUST(image_element->set_attribute(HTML::AttributeNames::width, String::formatted("{}", width)));
     }
 
     // 4. If height is given, then set an attribute value for img using "height" and height.
     if (vm.argument_count() > 1) {
         u32 height = TRY(vm.argument(1).to_u32(vm));
-        image_element->set_attribute(HTML::AttributeNames::height, String::formatted("{}", height));
+        MUST(image_element->set_attribute(HTML::AttributeNames::height, String::formatted("{}", height)));
     }
 
     // 5. Return img.

Userland/Libraries/LibWeb/Bindings/OptionConstructor.cpp

@@ -52,21 +52,21 @@ JS::ThrowCompletionOr<JS::Object*> OptionConstructor::construct(FunctionObject&)
         auto text = TRY(vm.argument(0).to_string(vm));
         if (!text.is_empty()) {
             auto new_text_node = vm.heap().allocate<DOM::Text>(realm, document, text);
-            option_element->append_child(*new_text_node);
+            MUST(option_element->append_child(*new_text_node));
         }
     }
 
     // 4. If value is given, then set an attribute value for option using "value" and value.
     if (vm.argument_count() > 1) {
         auto value = TRY(vm.argument(1).to_string(vm));
-        option_element->set_attribute(HTML::AttributeNames::value, value);
+        MUST(option_element->set_attribute(HTML::AttributeNames::value, value));
     }
 
     // 5. If defaultSelected is true, then set an attribute value for option using "selected" and the empty string.
     if (vm.argument_count() > 2) {
         auto default_selected = vm.argument(2).to_boolean();
         if (default_selected) {
-            option_element->set_attribute(HTML::AttributeNames::selected, "");
+            MUST(option_element->set_attribute(HTML::AttributeNames::selected, ""));
         }
     }
 

Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <LibWeb/Bindings/CSSStyleDeclarationPrototype.h>
+#include <LibWeb/Bindings/ExceptionOrUtils.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/CSSStyleDeclaration.h>
 #include <LibWeb/CSS/Parser/Parser.h>
@@ -163,7 +164,7 @@ void ElementInlineCSSStyleDeclaration::update_style_attribute()
     m_updating = true;
 
     // 5. Set an attribute value for owner node using "style" and the result of serializing declaration block.
-    m_element->set_attribute(HTML::AttributeNames::style, serialized());
+    MUST(m_element->set_attribute(HTML::AttributeNames::style, serialized()));
 
     // 6. Unset declaration block’s updating flag.
     m_updating = false;
@@ -346,15 +347,16 @@ JS::ThrowCompletionOr<JS::Value> CSSStyleDeclaration::internal_get(JS::PropertyK
 
 JS::ThrowCompletionOr<bool> CSSStyleDeclaration::internal_set(JS::PropertyKey const& name, JS::Value value, JS::Value receiver)
 {
+    auto& vm = this->vm();
     if (!name.is_string())
         return Base::internal_set(name, value, receiver);
     auto property_id = property_id_from_name(name.to_string());
     if (property_id == CSS::PropertyID::Invalid)
         return Base::internal_set(name, value, receiver);
 
-    auto css_text = TRY(value.to_string(vm()));
+    auto css_text = TRY(value.to_string(vm));
 
-    set_property(property_id, css_text);
+    TRY(Bindings::throw_dom_exception_if_needed(vm, [&] { return set_property(property_id, css_text); }));
     return true;
 }
 

Userland/Libraries/LibWeb/DOM/CharacterData.cpp

@@ -78,25 +78,25 @@ WebIDL::ExceptionOr<void> CharacterData::replace_data(size_t offset, size_t coun
     // 8. For each live range whose start node is node and start offset is greater than offset but less than or equal to offset plus count, set its start offset to offset.
     for (auto& range : Range::live_ranges()) {
         if (range->start_container() == this && range->start_offset() > offset && range->start_offset() <= (offset + count))
-            range->set_start(*range->start_container(), offset);
+            TRY(range->set_start(*range->start_container(), offset));
     }
 
     // 9. For each live range whose end node is node and end offset is greater than offset but less than or equal to offset plus count, set its end offset to offset.
     for (auto& range : Range::live_ranges()) {
         if (range->end_container() == this && range->end_offset() > offset && range->end_offset() <= (offset + count))
-            range->set_end(*range->end_container(), range->end_offset());
+            TRY(range->set_end(*range->end_container(), range->end_offset()));
     }
 
     // 10. For each live range whose start node is node and start offset is greater than offset plus count, increase its start offset by data’s length and decrease it by count.
     for (auto& range : Range::live_ranges()) {
         if (range->start_container() == this && range->start_offset() > (offset + count))
-            range->set_start(*range->start_container(), range->start_offset() + data.length() - count);
+            TRY(range->set_start(*range->start_container(), range->start_offset() + data.length() - count));
     }
 
     // 11. For each live range whose end node is node and end offset is greater than offset plus count, increase its end offset by data’s length and decrease it by count.
     for (auto& range : Range::live_ranges()) {
         if (range->end_container() == this && range->end_offset() > (offset + count))
-            range->set_end(*range->end_container(), range->end_offset() + data.length() - count);
+            TRY(range->set_end(*range->end_container(), range->end_offset() + data.length() - count));
     }
 
     // 12. If node’s parent is non-null, then run the children changed steps for node’s parent.

Userland/Libraries/LibWeb/DOM/DOMImplementation.cpp

@@ -51,10 +51,10 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<Document>> DOMImplementation::create_docume
         element = TRY(xml_document->create_element_ns(namespace_, qualified_name /* FIXME: and an empty dictionary */));
 
     if (doctype)
-        xml_document->append_child(*doctype);
+        TRY(xml_document->append_child(*doctype));
 
     if (element)
-        xml_document->append_child(*element);
+        TRY(xml_document->append_child(*element));
 
     xml_document->set_origin(document().origin());
 
@@ -78,24 +78,24 @@ JS::NonnullGCPtr<Document> DOMImplementation::create_html_document(String const&
 
     auto doctype = heap().allocate<DocumentType>(realm(), html_document);
     doctype->set_name("html");
-    html_document->append_child(*doctype);
+    MUST(html_document->append_child(*doctype));
 
     auto html_element = create_element(html_document, HTML::TagNames::html, Namespace::HTML);
-    html_document->append_child(html_element);
+    MUST(html_document->append_child(html_element));
 
     auto head_element = create_element(html_document, HTML::TagNames::head, Namespace::HTML);
-    html_element->append_child(head_element);
+    MUST(html_element->append_child(head_element));
 
     if (!title.is_null()) {
         auto title_element = create_element(html_document, HTML::TagNames::title, Namespace::HTML);
-        head_element->append_child(title_element);
+        MUST(head_element->append_child(title_element));
 
         auto text_node = heap().allocate<Text>(realm(), html_document, title);
-        title_element->append_child(*text_node);
+        MUST(title_element->append_child(*text_node));
     }
 
     auto body_element = create_element(html_document, HTML::TagNames::body, Namespace::HTML);
-    html_element->append_child(body_element);
+    MUST(html_element->append_child(body_element));
 
     html_document->set_origin(document().origin());
 

Userland/Libraries/LibWeb/DOM/DOMTokenList.cpp

@@ -227,7 +227,7 @@ void DOMTokenList::set_value(String value)
     if (!associated_element)
         return;
 
-    associated_element->set_attribute(m_associated_attribute, move(value));
+    MUST(associated_element->set_attribute(m_associated_attribute, move(value)));
 }
 
 WebIDL::ExceptionOr<void> DOMTokenList::validate_token(StringView token) const
@@ -251,7 +251,7 @@ void DOMTokenList::run_update_steps()
         return;
 
     // 2. Set an attribute value for the associated element using associated attribute’s local name and the result of running the ordered set serializer for token set.
-    associated_element->set_attribute(m_associated_attribute, value());
+    MUST(associated_element->set_attribute(m_associated_attribute, value()));
 }
 
 JS::Value DOMTokenList::item_value(size_t index) const

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -415,7 +415,7 @@ WebIDL::ExceptionOr<void> Document::run_the_document_write_steps(String input)
             return {};
 
         // 2. Run the document open steps with document.
-        open();
+        TRY(open());
     }
 
     // 5. Insert input into the input stream just before the insertion point.
@@ -664,11 +664,11 @@ void Document::set_title(String const& title)
     JS::GCPtr<HTML::HTMLTitleElement> title_element = head_element->first_child_of_type<HTML::HTMLTitleElement>();
     if (!title_element) {
         title_element = &static_cast<HTML::HTMLTitleElement&>(*create_element(HTML::TagNames::title).release_value());
-        head_element->append_child(*title_element);
+        MUST(head_element->append_child(*title_element));
     }
 
     title_element->remove_all_children(true);
-    title_element->append_child(*heap().allocate<Text>(realm(), *this, title));
+    MUST(title_element->append_child(*heap().allocate<Text>(realm(), *this, title)));
 
     if (auto* page = this->page()) {
         if (browsing_context() == &page->top_level_browsing_context())

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -399,13 +399,13 @@ void Node::insert_before(JS::NonnullGCPtr<Node> node, JS::GCPtr<Node> child, boo
         // 1. For each live range whose start node is parent and start offset is greater than child’s index, increase its start offset by count.
         for (auto& range : Range::live_ranges()) {
             if (range->start_container() == this && range->start_offset() > child->index())
-                range->set_start(*range->start_container(), range->start_offset() + count);
+                MUST(range->set_start(*range->start_container(), range->start_offset() + count));
         }
 
         // 2. For each live range whose end node is parent and end offset is greater than child’s index, increase its end offset by count.
         for (auto& range : Range::live_ranges()) {
             if (range->end_container() == this && range->end_offset() > child->index())
-                range->set_end(*range->end_container(), range->end_offset() + count);
+                MUST(range->set_end(*range->end_container(), range->end_offset() + count));
         }
     }
 
@@ -525,25 +525,25 @@ void Node::remove(bool suppress_observers)
     // 4. For each live range whose start node is an inclusive descendant of node, set its start to (parent, index).
     for (auto& range : Range::live_ranges()) {
         if (range->start_container()->is_inclusive_descendant_of(*this))
-            range->set_start(*parent, index);
+            MUST(range->set_start(*parent, index));
     }
 
     // 5. For each live range whose end node is an inclusive descendant of node, set its end to (parent, index).
     for (auto& range : Range::live_ranges()) {
         if (range->end_container()->is_inclusive_descendant_of(*this))
-            range->set_end(*parent, index);
+            MUST(range->set_end(*parent, index));
     }
 
     // 6. For each live range whose start node is parent and start offset is greater than index, decrease its start offset by 1.
     for (auto& range : Range::live_ranges()) {
         if (range->start_container() == parent && range->start_offset() > index)
-            range->set_start(*range->start_container(), range->start_offset() - 1);
+            MUST(range->set_start(*range->start_container(), range->start_offset() - 1));
     }
 
     // 7. For each live range whose end node is parent and end offset is greater than index, decrease its end offset by 1.
     for (auto& range : Range::live_ranges()) {
         if (range->end_container() == parent && range->end_offset() > index)
-            range->set_end(*range->end_container(), range->end_offset() - 1);
+            MUST(range->set_end(*range->end_container(), range->end_offset() - 1));
     }
 
     // 8. For each NodeIterator object iterator whose root’s node document is node’s node document, run the NodeIterator pre-removing steps given node and iterator.
@@ -721,7 +721,7 @@ JS::NonnullGCPtr<Node> Node::clone_node(Document* document, bool clone_children)
         element.for_each_attribute([&](auto& name, auto& value) {
             // 1. Let copyAttribute be a clone of attribute.
             // 2. Append copyAttribute to copy.
-            element_copy->set_attribute(name, value);
+            MUST(element_copy->set_attribute(name, value));
         });
         copy = move(element_copy);
 
@@ -790,7 +790,7 @@ JS::NonnullGCPtr<Node> Node::clone_node(Document* document, bool clone_children)
     // 6. If the clone children flag is set, clone all the children of node and append them to copy, with document as specified and the clone children flag being set.
     if (clone_children) {
         for_each_child([&](auto& child) {
-            copy->append_child(child.clone_node(document, true));
+            MUST(copy->append_child(child.clone_node(document, true)));
         });
     }
 

Userland/Libraries/LibWeb/DOM/Range.cpp

@@ -578,10 +578,10 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::extract()
         verify_cast<CharacterData>(*clone).set_data(move(result));
 
         // 3. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
 
         // 4. Replace data with node original start node, offset original start offset, count original end offset minus original start offset, and data the empty string.
-        static_cast<CharacterData&>(*original_start_node).replace_data(original_start_offset, original_end_offset - original_start_offset, "");
+        TRY(static_cast<CharacterData&>(*original_start_node).replace_data(original_start_offset, original_end_offset - original_start_offset, ""));
 
         // 5. Return fragment.
         return JS::NonnullGCPtr(*fragment);
@@ -668,10 +668,10 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::extract()
         verify_cast<CharacterData>(*clone).set_data(move(result));
 
         // 3. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
 
         // 4. Replace data with node original start node, offset original start offset, count original start node’s length minus original start offset, and data the empty string.
-        static_cast<CharacterData&>(*original_start_node).replace_data(original_start_offset, original_start_node->length() - original_start_offset, "");
+        TRY(static_cast<CharacterData&>(*original_start_node).replace_data(original_start_offset, original_start_node->length() - original_start_offset, ""));
     }
     // 16. Otherwise, if first partially contained child is not null:
     else if (first_partially_contained_child) {
@@ -679,7 +679,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::extract()
         auto clone = first_partially_contained_child->clone_node();
 
         // 2. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
 
         // 3. Let subrange be a new live range whose start is (original start node, original start offset) and whose end is (first partially contained child, first partially contained child’s length).
         auto subrange = Range::create(original_start_node, original_start_offset, *first_partially_contained_child, first_partially_contained_child->length());
@@ -688,12 +688,12 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::extract()
         auto subfragment = TRY(subrange->extract());
 
         // 5. Append subfragment to clone.
-        clone->append_child(subfragment);
+        TRY(clone->append_child(subfragment));
     }
 
     // 17. For each contained child in contained children, append contained child to fragment.
     for (auto& contained_child : contained_children) {
-        fragment->append_child(contained_child);
+        TRY(fragment->append_child(contained_child));
     }
 
     // 18. If last partially contained child is a CharacterData node, then:
@@ -706,10 +706,10 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::extract()
         verify_cast<CharacterData>(*clone).set_data(move(result));
 
         // 3. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
 
         // 4. Replace data with node original end node, offset 0, count original end offset, and data the empty string.
-        verify_cast<CharacterData>(*original_end_node).replace_data(0, original_end_offset, "");
+        TRY(verify_cast<CharacterData>(*original_end_node).replace_data(0, original_end_offset, ""));
     }
     // 19. Otherwise, if last partially contained child is not null:
     else if (last_partially_contained_child) {
@@ -717,7 +717,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::extract()
         auto clone = last_partially_contained_child->clone_node();
 
         // 2. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
 
         // 3. Let subrange be a new live range whose start is (last partially contained child, 0) and whose end is (original end node, original end offset).
         auto subrange = Range::create(*last_partially_contained_child, 0, original_end_node, original_end_offset);
@@ -726,12 +726,12 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::extract()
         auto subfragment = TRY(subrange->extract());
 
         // 5. Append subfragment to clone.
-        clone->append_child(subfragment);
+        TRY(clone->append_child(subfragment));
     }
 
     // 20. Set range’s start and end to (new node, new offset).
-    set_start(*new_node, new_offset);
-    set_end(*new_node, new_offset);
+    TRY(set_start(*new_node, new_offset));
+    TRY(set_end(*new_node, new_offset));
 
     // 21. Return fragment.
     return JS::NonnullGCPtr(*fragment);
@@ -834,7 +834,7 @@ WebIDL::ExceptionOr<void> Range::insert(JS::NonnullGCPtr<Node> node)
 
     // 13. If range is collapsed, then set range’s end to (parent, newOffset).
     if (collapsed())
-        set_end(*parent, new_offset);
+        TRY(set_end(*parent, new_offset));
 
     return {};
 }
@@ -907,7 +907,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::clone_the_content
         verify_cast<CharacterData>(*clone).set_data(move(result));
 
         // 3. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
 
         // 4. Return fragment.
         return JS::NonnullGCPtr(*fragment);
@@ -972,7 +972,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::clone_the_content
         verify_cast<CharacterData>(*clone).set_data(move(result));
 
         // 3. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
     }
     // 14. Otherwise, if first partially contained child is not null:
     else if (first_partially_contained_child) {
@@ -980,7 +980,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::clone_the_content
         auto clone = first_partially_contained_child->clone_node();
 
         // 2. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
 
         // 3. Let subrange be a new live range whose start is (original start node, original start offset) and whose end is (first partially contained child, first partially contained child’s length).
         auto subrange = Range::create(original_start_node, original_start_offset, *first_partially_contained_child, first_partially_contained_child->length());
@@ -989,7 +989,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::clone_the_content
         auto subfragment = TRY(subrange->clone_the_contents());
 
         // 5. Append subfragment to clone.
-        clone->append_child(subfragment);
+        TRY(clone->append_child(subfragment));
     }
 
     // 15. For each contained child in contained children.
@@ -998,7 +998,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::clone_the_content
         auto clone = contained_child->clone_node(nullptr, true);
 
         // 2. Append clone to fragment.
-        fragment->append_child(move(clone));
+        TRY(fragment->append_child(move(clone)));
     }
 
     // 16. If last partially contained child is a CharacterData node, then:
@@ -1011,7 +1011,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::clone_the_content
         verify_cast<CharacterData>(*clone).set_data(move(result));
 
         // 3. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
     }
     // 17. Otherwise, if last partially contained child is not null:
     else if (last_partially_contained_child) {
@@ -1019,7 +1019,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::clone_the_content
         auto clone = last_partially_contained_child->clone_node();
 
         // 2. Append clone to fragment.
-        fragment->append_child(clone);
+        TRY(fragment->append_child(clone));
 
         // 3. Let subrange be a new live range whose start is (last partially contained child, 0) and whose end is (original end node, original end offset).
         auto subrange = Range::create(*last_partially_contained_child, 0, original_end_node, original_end_offset);
@@ -1028,7 +1028,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<DocumentFragment>> Range::clone_the_content
         auto subfragment = TRY(subrange->clone_the_contents());
 
         // 5. Append subfragment to clone.
-        clone->append_child(subfragment);
+        TRY(clone->append_child(subfragment));
     }
 
     // 18. Return fragment.
@@ -1097,8 +1097,8 @@ WebIDL::ExceptionOr<void> Range::delete_contents()
         TRY(static_cast<CharacterData&>(*original_end_node).replace_data(0, original_end_offset, ""));
 
     // 10. Set start and end to (new node, new offset).
-    set_start(*new_node, new_offset);
-    set_end(*new_node, new_offset);
+    TRY(set_start(*new_node, new_offset));
+    TRY(set_end(*new_node, new_offset));
     return {};
 }
 

Userland/Libraries/LibWeb/DOM/Text.cpp

@@ -76,25 +76,25 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<Text>> Text::split_text(size_t offset)
         // 2. For each live range whose start node is node and start offset is greater than offset, set its start node to new node and decrease its start offset by offset.
         for (auto& range : Range::live_ranges()) {
             if (range->start_container() == this && range->start_offset() > offset)
-                range->set_start(*new_node, range->start_offset() - offset);
+                TRY(range->set_start(*new_node, range->start_offset() - offset));
         }
 
         // 3. For each live range whose end node is node and end offset is greater than offset, set its end node to new node and decrease its end offset by offset.
         for (auto& range : Range::live_ranges()) {
             if (range->end_container() == this && range->end_offset() > offset)
-                range->set_end(*new_node, range->end_offset() - offset);
+                TRY(range->set_end(*new_node, range->end_offset() - offset));
         }
 
         // 4. For each live range whose start node is parent and start offset is equal to the index of node plus 1, increase its start offset by 1.
         for (auto& range : Range::live_ranges()) {
             if (range->start_container() == this && range->start_offset() == index() + 1)
-                range->set_start(*range->start_container(), range->start_offset() + 1);
+                TRY(range->set_start(*range->start_container(), range->start_offset() + 1));
         }
 
         // 5. For each live range whose end node is parent and end offset is equal to the index of node plus 1, increase its end offset by 1.
         for (auto& range : Range::live_ranges()) {
             if (range->end_container() == parent.ptr() && range->end_offset() == index() + 1) {
-                range->set_end(*range->end_container(), range->end_offset() + 1);
+                TRY(range->set_end(*range->end_container(), range->end_offset() + 1));
             }
         }
     }

Userland/Libraries/LibWeb/Fetch/Request.cpp

@@ -420,7 +420,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<Request>> Request::construct_impl(JS::Realm
         }
         // 5. Otherwise, fill this’s headers with headers.
         else {
-            request_object->headers()->fill(headers.get<HeadersInit>());
+            TRY(request_object->headers()->fill(headers.get<HeadersInit>()));
         }
     }
 

Userland/Libraries/LibWeb/Fetch/Response.cpp

@@ -96,7 +96,7 @@ WebIDL::ExceptionOr<void> Response::initialize_response(ResponseInit const& init
 
     // 5. If init["headers"] exists, then fill response’s headers with init["headers"].
     if (init.headers.has_value())
-        m_headers->fill(*init.headers);
+        TRY(m_headers->fill(*init.headers));
 
     // 6. If body was given, then:
     if (body.has_value()) {

Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp

@@ -188,9 +188,9 @@ JS::NonnullGCPtr<BrowsingContext> BrowsingContext::create_a_new_browsing_context
 
     // 18. Ensure that document has a single child html node, which itself has two empty child nodes: a head element, and a body element.
     auto html_node = document->create_element(HTML::TagNames::html).release_value();
-    html_node->append_child(document->create_element(HTML::TagNames::head).release_value());
-    html_node->append_child(document->create_element(HTML::TagNames::body).release_value());
-    document->append_child(html_node);
+    MUST(html_node->append_child(document->create_element(HTML::TagNames::head).release_value()));
+    MUST(html_node->append_child(document->create_element(HTML::TagNames::body).release_value()));
+    MUST(document->append_child(html_node));
 
     // 19. Set the active document of browsingContext to document.
     browsing_context->set_active_document(*document);
@@ -952,7 +952,7 @@ WebIDL::ExceptionOr<void> BrowsingContext::navigate(
         && resource->url().equals(active_document()->url(), AK::URL::ExcludeFragment::Yes)
         && !resource->url().fragment().is_null()) {
         // 1. Navigate to a fragment given browsingContext, resource's URL, historyHandling, and navigationId.
-        navigate_to_a_fragment(resource->url(), history_handling, *navigation_id);
+        TRY(navigate_to_a_fragment(resource->url(), history_handling, *navigation_id));
 
         // 2. Return.
         return {};

Userland/Libraries/LibWeb/HTML/BrowsingContextContainer.cpp

@@ -220,7 +220,7 @@ void BrowsingContextContainer::navigate_an_iframe_or_frame(JS::NonnullGCPtr<Fetc
     //    FIXME: and processResponseEndOfBody set to reportFrameTiming.
     auto* source_browsing_context = document().browsing_context();
     VERIFY(source_browsing_context);
-    m_nested_browsing_context->navigate(resource, *source_browsing_context, false, history_handling);
+    MUST(m_nested_browsing_context->navigate(resource, *source_browsing_context, false, history_handling));
 }
 
 }

Userland/Libraries/LibWeb/HTML/DOMParser.cpp

@@ -64,7 +64,7 @@ JS::NonnullGCPtr<DOM::Document> DOMParser::parse_from_string(String const& strin
             auto root = DOM::create_element(*document, "parsererror", "http://www.mozilla.org/newlayout/xml/parsererror.xml");
             // FIXME: 3. Optionally, add attributes or children to root to describe the nature of the parsing error.
             // 4. Append root to document.
-            document->append_child(*root);
+            MUST(document->append_child(*root));
         }
     }
 

Userland/Libraries/LibWeb/HTML/DOMStringMap.cpp

@@ -144,7 +144,7 @@ WebIDL::ExceptionOr<void> DOMStringMap::set_value_of_new_named_property(String c
     // FIXME: 4. If name does not match the XML Name production, throw an "InvalidCharacterError" DOMException.
 
     // 5. Set an attribute value for the DOMStringMap's associated element using name and value.
-    m_associated_element->set_attribute(data_name, value);
+    MUST(m_associated_element->set_attribute(data_name, value));
 
     return {};
 }

Userland/Libraries/LibWeb/HTML/HTMLAnchorElement.cpp

@@ -36,7 +36,7 @@ String HTMLAnchorElement::hyperlink_element_utils_href() const
 
 void HTMLAnchorElement::set_hyperlink_element_utils_href(String href)
 {
-    set_attribute(HTML::AttributeNames::href, move(href));
+    MUST(set_attribute(HTML::AttributeNames::href, move(href)));
 }
 
 void HTMLAnchorElement::run_activation_behavior(Web::DOM::Event const&)

Userland/Libraries/LibWeb/HTML/HTMLAreaElement.cpp

@@ -32,7 +32,7 @@ String HTMLAreaElement::hyperlink_element_utils_href() const
 
 void HTMLAreaElement::set_hyperlink_element_utils_href(String href)
 {
-    set_attribute(HTML::AttributeNames::href, move(href));
+    MUST(set_attribute(HTML::AttributeNames::href, move(href)));
 }
 
 }

Userland/Libraries/LibWeb/HTML/HTMLBaseElement.cpp

@@ -91,7 +91,7 @@ String HTMLBaseElement::href() const
 void HTMLBaseElement::set_href(String const& href)
 {
     // The href IDL attribute, on setting, must set the href content attribute to the given new value.
-    set_attribute(AttributeNames::href, href);
+    MUST(set_attribute(AttributeNames::href, href));
 }
 
 }

Userland/Libraries/LibWeb/HTML/HTMLButtonElement.cpp

@@ -83,7 +83,7 @@ HTMLButtonElement::TypeAttributeState HTMLButtonElement::type_state() const
 
 void HTMLButtonElement::set_type(String const& type)
 {
-    set_attribute(HTML::AttributeNames::type, type);
+    MUST(set_attribute(HTML::AttributeNames::type, type));
 }
 
 }

Userland/Libraries/LibWeb/HTML/HTMLCanvasElement.cpp

@@ -66,14 +66,14 @@ void HTMLCanvasElement::reset_context_to_default_state()
 
 void HTMLCanvasElement::set_width(unsigned value)
 {
-    set_attribute(HTML::AttributeNames::width, String::number(value));
+    MUST(set_attribute(HTML::AttributeNames::width, String::number(value)));
     m_bitmap = nullptr;
     reset_context_to_default_state();
 }
 
 void HTMLCanvasElement::set_height(unsigned value)
 {
-    set_attribute(HTML::AttributeNames::height, String::number(value));
+    MUST(set_attribute(HTML::AttributeNames::height, String::number(value)));
     m_bitmap = nullptr;
     reset_context_to_default_state();
 }

Userland/Libraries/LibWeb/HTML/HTMLElement.cpp

@@ -101,11 +101,11 @@ WebIDL::ExceptionOr<void> HTMLElement::set_content_editable(String const& conten
         return {};
     }
     if (content_editable.equals_ignoring_case("true"sv)) {
-        set_attribute(HTML::AttributeNames::contenteditable, "true");
+        MUST(set_attribute(HTML::AttributeNames::contenteditable, "true"));
         return {};
     }
     if (content_editable.equals_ignoring_case("false"sv)) {
-        set_attribute(HTML::AttributeNames::contenteditable, "false");
+        MUST(set_attribute(HTML::AttributeNames::contenteditable, "false"));
         return {};
     }
     return WebIDL::SyntaxError::create(realm(), "Invalid contentEditable value, must be 'true', 'false', or 'inherit'");
@@ -114,7 +114,7 @@ WebIDL::ExceptionOr<void> HTMLElement::set_content_editable(String const& conten
 void HTMLElement::set_inner_text(StringView text)
 {
     remove_all_children();
-    append_child(document().create_text_node(text));
+    MUST(append_child(document().create_text_node(text)));
 
     set_needs_style_update(true);
 }

Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp

@@ -120,7 +120,7 @@ unsigned HTMLImageElement::width() const
 
 void HTMLImageElement::set_width(unsigned width)
 {
-    set_attribute(HTML::AttributeNames::width, String::number(width));
+    MUST(set_attribute(HTML::AttributeNames::width, String::number(width)));
 }
 
 // https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-height
@@ -148,7 +148,7 @@ unsigned HTMLImageElement::height() const
 
 void HTMLImageElement::set_height(unsigned height)
 {
-    set_attribute(HTML::AttributeNames::height, String::number(height));
+    MUST(set_attribute(HTML::AttributeNames::height, String::number(height)));
 }
 
 // https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-naturalwidth

Userland/Libraries/LibWeb/HTML/HTMLInputElement.cpp

@@ -340,12 +340,12 @@ void HTMLInputElement::create_shadow_tree_if_needed()
     if (initial_value.is_null())
         initial_value = String::empty();
     auto element = document().create_element(HTML::TagNames::div).release_value();
-    element->set_attribute(HTML::AttributeNames::style, "white-space: pre; padding-top: 1px; padding-bottom: 1px; padding-left: 2px; padding-right: 2px");
+    MUST(element->set_attribute(HTML::AttributeNames::style, "white-space: pre; padding-top: 1px; padding-bottom: 1px; padding-left: 2px; padding-right: 2px"));
     m_text_node = heap().allocate<DOM::Text>(realm(), document(), initial_value);
     m_text_node->set_always_editable(m_type != TypeAttributeState::FileUpload);
     m_text_node->set_owner_input_element({}, *this);
-    element->append_child(*m_text_node);
-    shadow_root->append_child(move(element));
+    MUST(element->append_child(*m_text_node));
+    MUST(shadow_root->append_child(move(element)));
     set_shadow_root(move(shadow_root));
 }
 
@@ -418,7 +418,7 @@ String HTMLInputElement::type() const
 
 void HTMLInputElement::set_type(String const& type)
 {
-    set_attribute(HTML::AttributeNames::type, type);
+    MUST(set_attribute(HTML::AttributeNames::type, type));
 }
 
 // https://html.spec.whatwg.org/multipage/input.html#value-sanitization-algorithm

Userland/Libraries/LibWeb/HTML/HTMLObjectElement.h

@@ -35,7 +35,7 @@ public:
     virtual void parse_attribute(FlyString const& name, String const& value) override;
 
     String data() const;
-    void set_data(String const& data) { set_attribute(HTML::AttributeNames::data, data); }
+    void set_data(String const& data) { MUST(set_attribute(HTML::AttributeNames::data, data)); }
 
     String type() const { return attribute(HTML::AttributeNames::type); }
 

Userland/Libraries/LibWeb/HTML/HTMLOptionElement.cpp

@@ -72,7 +72,7 @@ String HTMLOptionElement::value() const
 // https://html.spec.whatwg.org/multipage/form-elements.html#dom-option-value
 void HTMLOptionElement::set_value(String value)
 {
-    set_attribute(HTML::AttributeNames::value, value);
+    MUST(set_attribute(HTML::AttributeNames::value, value));
 }
 
 static void concatenate_descendants_text_content(DOM::Node const* node, StringBuilder& builder)

Userland/Libraries/LibWeb/HTML/HTMLProgressElement.cpp

@@ -67,7 +67,7 @@ void HTMLProgressElement::set_value(double value)
     if (value < 0)
         return;
 
-    set_attribute(HTML::AttributeNames::value, String::number(value));
+    MUST(set_attribute(HTML::AttributeNames::value, String::number(value)));
     progress_position_updated();
 }
 
@@ -93,7 +93,7 @@ void HTMLProgressElement::set_max(double value)
     if (value <= 0)
         return;
 
-    set_attribute(HTML::AttributeNames::max, String::number(value));
+    MUST(set_attribute(HTML::AttributeNames::max, String::number(value)));
     progress_position_updated();
 }
 

Userland/Libraries/LibWeb/HTML/HTMLTableCellElement.cpp

@@ -56,7 +56,7 @@ unsigned int HTMLTableCellElement::col_span() const
 
 void HTMLTableCellElement::set_col_span(unsigned int value)
 {
-    set_attribute(HTML::AttributeNames::colspan, String::number(value));
+    MUST(set_attribute(HTML::AttributeNames::colspan, String::number(value)));
 }
 
 unsigned int HTMLTableCellElement::row_span() const
@@ -66,7 +66,7 @@ unsigned int HTMLTableCellElement::row_span() const
 
 void HTMLTableCellElement::set_row_span(unsigned int value)
 {
-    set_attribute(HTML::AttributeNames::rowspan, String::number(value));
+    MUST(set_attribute(HTML::AttributeNames::rowspan, String::number(value)));
 }
 
 }

Userland/Libraries/LibWeb/HTML/HTMLTableElement.cpp

@@ -61,7 +61,7 @@ void HTMLTableElement::set_caption(HTMLTableCaptionElement* caption)
     //        Currently the wrapper generator doesn't send us a nullable value
     delete_caption();
 
-    pre_insert(*caption, first_child());
+    MUST(pre_insert(*caption, first_child()));
 }
 
 JS::NonnullGCPtr<HTMLTableCaptionElement> HTMLTableElement::create_caption()
@@ -72,7 +72,7 @@ JS::NonnullGCPtr<HTMLTableCaptionElement> HTMLTableElement::create_caption()
     }
 
     auto caption = DOM::create_element(document(), TagNames::caption, Namespace::HTML);
-    pre_insert(caption, first_child());
+    MUST(pre_insert(caption, first_child()));
     return static_cast<HTMLTableCaptionElement&>(*caption);
 }
 
@@ -127,7 +127,7 @@ WebIDL::ExceptionOr<void> HTMLTableElement::set_t_head(HTMLTableSectionElement*
         break;
     }
 
-    pre_insert(*thead, child_to_append_after);
+    TRY(pre_insert(*thead, child_to_append_after));
 
     return {};
 }
@@ -158,7 +158,7 @@ JS::NonnullGCPtr<HTMLTableSectionElement> HTMLTableElement::create_t_head()
         break;
     }
 
-    pre_insert(thead, child_to_append_after);
+    MUST(pre_insert(thead, child_to_append_after));
 
     return static_cast<HTMLTableSectionElement&>(*thead);
 }
@@ -197,7 +197,7 @@ WebIDL::ExceptionOr<void> HTMLTableElement::set_t_foot(HTMLTableSectionElement*
     delete_t_foot();
 
     // We insert the new tfoot at the end of the table
-    append_child(*tfoot);
+    TRY(append_child(*tfoot));
 
     return {};
 }
@@ -209,7 +209,7 @@ JS::NonnullGCPtr<HTMLTableSectionElement> HTMLTableElement::create_t_foot()
         return *maybe_tfoot;
 
     auto tfoot = DOM::create_element(document(), TagNames::tfoot, Namespace::HTML);
-    append_child(tfoot);
+    MUST(append_child(tfoot));
     return static_cast<HTMLTableSectionElement&>(*tfoot);
 }
 
@@ -247,7 +247,7 @@ JS::NonnullGCPtr<HTMLTableSectionElement> HTMLTableElement::create_t_body()
         }
     }
 
-    pre_insert(tbody, child_to_append_after);
+    MUST(pre_insert(tbody, child_to_append_after));
 
     return static_cast<HTMLTableSectionElement&>(*tbody);
 }
@@ -291,14 +291,14 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<HTMLTableRowElement>> HTMLTableElement::ins
     auto& tr = static_cast<HTMLTableRowElement&>(*DOM::create_element(document(), TagNames::tr, Namespace::HTML));
     if (rows_length == 0 && !has_child_of_type<HTMLTableRowElement>()) {
         auto tbody = DOM::create_element(document(), TagNames::tbody, Namespace::HTML);
-        tbody->append_child(tr);
-        append_child(tbody);
+        TRY(tbody->append_child(tr));
+        TRY(append_child(tbody));
     } else if (rows_length == 0) {
         auto tbody = last_child_of_type<HTMLTableRowElement>();
-        tbody->append_child(tr);
+        TRY(tbody->append_child(tr));
     } else if (index == -1 || index == (long)rows_length) {
         auto parent_of_last_tr = rows->item(rows_length - 1)->parent_element();
-        parent_of_last_tr->append_child(tr);
+        TRY(parent_of_last_tr->append_child(tr));
     } else {
         rows->item(index)->parent_element()->insert_before(tr, rows->item(index));
     }

Userland/Libraries/LibWeb/HTML/HTMLTableSectionElement.cpp

@@ -50,7 +50,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<HTMLTableRowElement>> HTMLTableSectionEleme
 
     // 3. If index is −1 or equal to the number of items in the rows collection, then append table row to this element.
     if (index == -1 || index == rows_collection_size)
-        append_child(table_row);
+        TRY(append_child(table_row));
     // 4. Otherwise, insert table row as a child of this element, immediately before the index-th tr element in the rows collection.
     else
         table_row.insert_before(*this, rows_collection->item(index));

Userland/Libraries/LibWeb/HTML/HTMLTemplateElement.cpp

@@ -53,7 +53,7 @@ void HTMLTemplateElement::cloned(Node& copy, bool clone_children)
         auto cloned_child = child.clone_node(&template_clone.content()->document(), true);
 
         // FIXME: Should this use TreeNode::append_child instead?
-        template_clone.content()->append_child(cloned_child);
+        MUST(template_clone.content()->append_child(cloned_child));
     });
 }
 

Userland/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp

@@ -465,7 +465,7 @@ void HTMLParser::handle_initial(HTMLToken& token)
 
     if (token.is_comment()) {
         auto comment = realm().heap().allocate<DOM::Comment>(realm(), document(), token.comment());
-        document().append_child(*comment);
+        MUST(document().append_child(*comment));
         return;
     }
 
@@ -474,7 +474,7 @@ void HTMLParser::handle_initial(HTMLToken& token)
         doctype->set_name(token.doctype_data().name);
         doctype->set_public_id(token.doctype_data().public_identifier);
         doctype->set_system_id(token.doctype_data().system_identifier);
-        document().append_child(*doctype);
+        MUST(document().append_child(*doctype));
         document().set_quirks_mode(which_quirks_mode(token));
         m_insertion_mode = InsertionMode::BeforeHTML;
         return;
@@ -495,7 +495,7 @@ void HTMLParser::handle_before_html(HTMLToken& token)
 
     if (token.is_comment()) {
         auto comment = realm().heap().allocate<DOM::Comment>(realm(), document(), token.comment());
-        document().append_child(*comment);
+        MUST(document().append_child(*comment));
         return;
     }
 
@@ -505,7 +505,7 @@ void HTMLParser::handle_before_html(HTMLToken& token)
 
     if (token.is_start_tag() && token.tag_name() == HTML::TagNames::html) {
         auto element = create_element_for(token, Namespace::HTML, document());
-        document().append_child(*element);
+        MUST(document().append_child(*element));
         m_stack_of_open_elements.push(move(element));
         m_insertion_mode = InsertionMode::BeforeHead;
         return;
@@ -522,7 +522,7 @@ void HTMLParser::handle_before_html(HTMLToken& token)
 
 AnythingElse:
     auto element = create_element(document(), HTML::TagNames::html, Namespace::HTML);
-    document().append_child(*element);
+    MUST(document().append_child(*element));
     m_stack_of_open_elements.push(element);
     // FIXME: If the Document is being loaded as part of navigation of a browsing context, then: run the application cache selection algorithm with no manifest, passing it the Document object.
     m_insertion_mode = InsertionMode::BeforeHead;
@@ -627,7 +627,7 @@ JS::NonnullGCPtr<DOM::Element> HTMLParser::create_element_for(HTMLToken const& t
     // 10. Append each attribute in the given token to element.
     // FIXME: This isn't the exact `append` the spec is talking about.
     token.for_each_attribute([&](auto& attribute) {
-        element->set_attribute(attribute.local_name, attribute.value);
+        MUST(element->set_attribute(attribute.local_name, attribute.value));
         return IterationDecision::Continue;
     });
 
@@ -930,7 +930,7 @@ DOM::Text* HTMLParser::find_character_insertion_node()
     if (adjusted_insertion_location.parent->last_child() && adjusted_insertion_location.parent->last_child()->is_text())
         return verify_cast<DOM::Text>(adjusted_insertion_location.parent->last_child());
     auto new_text_node = realm().heap().allocate<DOM::Text>(realm(), document(), "");
-    adjusted_insertion_location.parent->append_child(*new_text_node);
+    MUST(adjusted_insertion_location.parent->append_child(*new_text_node));
     return new_text_node;
 }
 
@@ -1055,7 +1055,7 @@ void HTMLParser::handle_after_body(HTMLToken& token)
 
     if (token.is_comment()) {
         auto& insertion_location = m_stack_of_open_elements.first();
-        insertion_location.append_child(*realm().heap().allocate<DOM::Comment>(realm(), document(), token.comment()));
+        MUST(insertion_location.append_child(*realm().heap().allocate<DOM::Comment>(realm(), document(), token.comment())));
         return;
     }
 
@@ -1092,7 +1092,7 @@ void HTMLParser::handle_after_after_body(HTMLToken& token)
 {
     if (token.is_comment()) {
         auto comment = realm().heap().allocate<DOM::Comment>(realm(), document(), token.comment());
-        document().append_child(*comment);
+        MUST(document().append_child(*comment));
         return;
     }
 
@@ -1311,7 +1311,7 @@ HTMLParser::AdoptionAgencyAlgorithmOutcome HTMLParser::run_the_adoption_agency_a
             }
 
             // 8. Append last node to node.
-            node->append_child(*last_node);
+            MUST(node->append_child(*last_node));
 
             // 9. Set last node to node.
             last_node = node;
@@ -1329,10 +1329,10 @@ HTMLParser::AdoptionAgencyAlgorithmOutcome HTMLParser::run_the_adoption_agency_a
 
         // 16. Take all of the child nodes of furthest block and append them to the element created in the last step.
         for (auto& child : furthest_block->children_as_vector())
-            element->append_child(furthest_block->remove_child(*child).release_value());
+            MUST(element->append_child(furthest_block->remove_child(*child).release_value()));
 
         // 17. Append that new element to furthest block.
-        furthest_block->append_child(*element);
+        MUST(furthest_block->append_child(*element));
 
         // 18. Remove formatting element from the list of active formatting elements,
         //     and insert the new element into the list of active formatting elements at the position of the aforementioned bookmark.
@@ -1481,7 +1481,7 @@ void HTMLParser::handle_in_body(HTMLToken& token)
             return;
         token.for_each_attribute([&](auto& attribute) {
             if (!current_node().has_attribute(attribute.local_name))
-                current_node().set_attribute(attribute.local_name, attribute.value);
+                MUST(current_node().set_attribute(attribute.local_name, attribute.value));
             return IterationDecision::Continue;
         });
         return;
@@ -1508,7 +1508,7 @@ void HTMLParser::handle_in_body(HTMLToken& token)
         auto& body_element = m_stack_of_open_elements.elements().at(1);
         token.for_each_attribute([&](auto& attribute) {
             if (!body_element->has_attribute(attribute.local_name))
-                body_element->set_attribute(attribute.local_name, attribute.value);
+                MUST(body_element->set_attribute(attribute.local_name, attribute.value));
             return IterationDecision::Continue;
         });
         return;
@@ -3126,7 +3126,7 @@ void HTMLParser::handle_after_after_frameset(HTMLToken& token)
 {
     if (token.is_comment()) {
         auto* comment = document().heap().allocate<DOM::Comment>(document().realm(), document(), token.comment());
-        document().append_child(*comment);
+        MUST(document().append_child(*comment));
         return;
     }
 
@@ -3446,7 +3446,7 @@ Vector<JS::Handle<DOM::Node>> HTMLParser::parse_html_fragment(DOM::Element& cont
     auto root = create_element(context_element.document(), HTML::TagNames::html, Namespace::HTML);
 
     // 6. Append the element root to the Document node created above.
-    temp_document->append_child(root);
+    MUST(temp_document->append_child(root));
 
     // 7. Set up the parser's stack of open elements so that it contains just the single element root.
     parser->m_stack_of_open_elements.push(root);
@@ -3475,7 +3475,7 @@ Vector<JS::Handle<DOM::Node>> HTMLParser::parse_html_fragment(DOM::Element& cont
     // 14. Return the child nodes of root, in tree order.
     Vector<JS::Handle<DOM::Node>> children;
     while (JS::GCPtr<DOM::Node> child = root->first_child()) {
-        root->remove_child(*child);
+        MUST(root->remove_child(*child));
         context_element.document().adopt_node(*child);
         children.append(JS::make_handle(*child));
     }

Userland/Libraries/LibWeb/HTML/Window.cpp

@@ -1423,7 +1423,9 @@ JS_DEFINE_NATIVE_FUNCTION(Window::post_message)
 {
     auto* impl = TRY(impl_from(vm));
     auto target_origin = TRY(vm.argument(1).to_string(vm));
-    impl->post_message_impl(vm.argument(0), target_origin);
+    TRY(Bindings::throw_dom_exception_if_needed(vm, [&] {
+        return impl->post_message_impl(vm.argument(0), target_origin);
+    }));
     return JS::js_undefined();
 }
 

Userland/Libraries/LibWeb/Loader/FrameLoader.cpp

@@ -104,23 +104,23 @@ static bool build_markdown_document(DOM::Document& document, ByteBuffer const& d
 static bool build_text_document(DOM::Document& document, ByteBuffer const& data)
 {
     auto html_element = document.create_element("html").release_value();
-    document.append_child(html_element);
+    MUST(document.append_child(html_element));
 
     auto head_element = document.create_element("head").release_value();
-    html_element->append_child(head_element);
+    MUST(html_element->append_child(head_element));
     auto title_element = document.create_element("title").release_value();
-    head_element->append_child(title_element);
+    MUST(head_element->append_child(title_element));
 
     auto title_text = document.create_text_node(document.url().basename());
-    title_element->append_child(title_text);
+    MUST(title_element->append_child(title_text));
 
     auto body_element = document.create_element("body").release_value();
-    html_element->append_child(body_element);
+    MUST(html_element->append_child(body_element));
 
     auto pre_element = document.create_element("pre").release_value();
-    body_element->append_child(pre_element);
+    MUST(body_element->append_child(pre_element));
 
-    pre_element->append_child(document.create_text_node(String::copy(data)));
+    MUST(pre_element->append_child(document.create_text_node(String::copy(data))));
     return true;
 }
 
@@ -135,23 +135,23 @@ static bool build_image_document(DOM::Document& document, ByteBuffer const& data
         return false;
 
     auto html_element = document.create_element("html").release_value();
-    document.append_child(html_element);
+    MUST(document.append_child(html_element));
 
     auto head_element = document.create_element("head").release_value();
-    html_element->append_child(head_element);
+    MUST(html_element->append_child(head_element));
     auto title_element = document.create_element("title").release_value();
-    head_element->append_child(title_element);
+    MUST(head_element->append_child(title_element));
 
     auto basename = LexicalPath::basename(document.url().path());
     auto title_text = document.heap().allocate<DOM::Text>(document.realm(), document, String::formatted("{} [{}x{}]", basename, bitmap->width(), bitmap->height()));
-    title_element->append_child(*title_text);
+    MUST(title_element->append_child(*title_text));
 
     auto body_element = document.create_element("body").release_value();
-    html_element->append_child(body_element);
+    MUST(html_element->append_child(body_element));
 
     auto image_element = document.create_element("img").release_value();
-    image_element->set_attribute(HTML::AttributeNames::src, document.url().to_string());
-    body_element->append_child(image_element);
+    MUST(image_element->set_attribute(HTML::AttributeNames::src, document.url().to_string()));
+    MUST(body_element->append_child(image_element));
 
     return true;
 }

Userland/Libraries/LibWeb/Selection/Selection.cpp

@@ -211,16 +211,16 @@ WebIDL::ExceptionOr<void> Selection::extend(JS::NonnullGCPtr<DOM::Node> node, un
 
     // 5. If node's root is not the same as the this's range's root, set the start newRange's start and end to newFocus.
     if (&node->root() != &m_range->start_container()->root()) {
-        new_range->set_start(new_focus_node, new_focus_offset);
+        TRY(new_range->set_start(new_focus_node, new_focus_offset));
     }
     // 6. Otherwise, if oldAnchor is before or equal to newFocus, set the start newRange's start to oldAnchor, then set its end to newFocus.
     else if (old_anchor_node.is_before(new_focus_node) || &old_anchor_node == new_focus_node.ptr()) {
-        new_range->set_end(new_focus_node, new_focus_offset);
+        TRY(new_range->set_end(new_focus_node, new_focus_offset));
     }
     // 7. Otherwise, set the start newRange's start to newFocus, then set its end to oldAnchor.
     else {
-        new_range->set_start(new_focus_node, new_focus_offset);
-        new_range->set_end(old_anchor_node, old_anchor_offset);
+        TRY(new_range->set_start(new_focus_node, new_focus_offset));
+        TRY(new_range->set_end(old_anchor_node, old_anchor_offset));
     }
 
     // 8. Set this's range to newRange.

Userland/Libraries/LibWeb/XML/XMLDocumentBuilder.cpp

@@ -70,13 +70,13 @@ void XMLDocumentBuilder::element_start(const XML::Name& name, HashMap<XML::Name,
     }
     if (HTML::TagNames::template_ == m_current_node->node_name()) {
         // When an XML parser would append a node to a template element, it must instead append it to the template element's template contents (a DocumentFragment node).
-        static_cast<HTML::HTMLTemplateElement&>(*m_current_node).content()->append_child(node);
+        MUST(static_cast<HTML::HTMLTemplateElement&>(*m_current_node).content()->append_child(node));
     } else {
-        m_current_node->append_child(node);
+        MUST(m_current_node->append_child(node));
     }
 
     for (auto& attribute : attributes)
-        node->set_attribute(attribute.key, attribute.value);
+        MUST(node->set_attribute(attribute.key, attribute.value));
 
     m_current_node = node.ptr();
 }
@@ -131,7 +131,7 @@ void XMLDocumentBuilder::text(String const& data)
         text_builder.clear();
     } else {
         auto node = m_document.create_text_node(data);
-        m_current_node->append_child(node);
+        MUST(m_current_node->append_child(node));
     }
 }
 
@@ -139,7 +139,7 @@ void XMLDocumentBuilder::comment(String const& data)
 {
     if (m_has_error)
         return;
-    m_document.append_child(m_document.create_comment(data));
+    MUST(m_document.append_child(m_document.create_comment(data)));
 }
 
 void XMLDocumentBuilder::document_end()