LibWeb: Implement CSS declaration block's "updating flag"

This flag is used to prevent reparsing the style attribute after it is
automatically updated after using the CSSOM API to mutate style.

Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.cpp

@@ -125,10 +125,22 @@ DOM::ExceptionOr<String> PropertyOwningCSSStyleDeclaration::remove_property(Prop
 // https://drafts.csswg.org/cssom/#update-style-attribute-for
 void ElementInlineCSSStyleDeclaration::update_style_attribute()
 {
+    // 1. Assert: declaration block’s computed flag is unset.
+    // NOTE: Unnecessary, only relevant for ResolvedCSSStyleDeclaration.
+
+    // 2. Let owner node be declaration block’s owner node.
+    // 3. If owner node is null, then return.
     if (!m_element)
         return;
 
+    // 4. Set declaration block’s updating flag.
+    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());
+
+    // 6. Unset declaration block’s updating flag.
+    m_updating = false;
 }
 
 // https://drafts.csswg.org/cssom/#set-a-css-declaration

Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.h

@@ -101,12 +101,17 @@ public:
     DOM::Element* element() { return m_element.ptr(); }
     const DOM::Element* element() const { return m_element.ptr(); }
 
+    bool is_updating() const { return m_updating; }
+
 private:
     explicit ElementInlineCSSStyleDeclaration(DOM::Element&, Vector<StyleProperty> properties, HashMap<String, StyleProperty> custom_properties);
 
     virtual void update_style_attribute() override;
 
     WeakPtr<DOM::Element> m_element;
+
+    // https://drafts.csswg.org/cssom/#cssstyledeclaration-updating-flag
+    bool m_updating { false };
 };
 
 }

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -293,6 +293,11 @@ RefPtr<Layout::Node> Element::create_layout_node_for_display_type(DOM::Document&
     TODO();
 }
 
+CSS::CSSStyleDeclaration const* Element::inline_style() const
+{
+    return m_inline_style;
+}
+
 void Element::parse_attribute(FlyString const& name, String const& value)
 {
     if (name == HTML::AttributeNames::class_) {
@@ -305,6 +310,9 @@ void Element::parse_attribute(FlyString const& name, String const& value)
         if (m_class_list)
             m_class_list->associated_attribute_changed(value);
     } else if (name == HTML::AttributeNames::style) {
+        // https://drafts.csswg.org/cssom/#ref-for-cssstyledeclaration-updating-flag
+        if (m_inline_style && m_inline_style->is_updating())
+            return;
         m_inline_style = parse_css_style_attribute(CSS::ParsingContext(document()), value, *this);
         set_needs_style_update(true);
     }

Userland/Libraries/LibWeb/DOM/Element.h

@@ -103,7 +103,7 @@ public:
     void set_computed_css_values(RefPtr<CSS::StyleProperties> style) { m_computed_css_values = move(style); }
     NonnullRefPtr<CSS::StyleProperties> resolved_css_values();
 
-    const CSS::CSSStyleDeclaration* inline_style() const { return m_inline_style; }
+    CSS::CSSStyleDeclaration const* inline_style() const;
 
     NonnullRefPtr<CSS::CSSStyleDeclaration> style_for_bindings();
 
@@ -152,7 +152,7 @@ private:
     String m_html_uppercased_qualified_name;
     NonnullRefPtr<NamedNodeMap> m_attributes;
 
-    RefPtr<CSS::CSSStyleDeclaration> m_inline_style;
+    RefPtr<CSS::ElementInlineCSSStyleDeclaration> m_inline_style;
 
     RefPtr<CSS::StyleProperties> m_computed_css_values;
     HashMap<FlyString, CSS::StyleProperty> m_custom_properties;