LibWeb: Add support for the <base> element changing the base URL

Used by Google seemingly almost all around account sign in and
management. The modern sign in page has this near the beginning:
```html
<base href="https://accounts.google.com">
```
All of the XHRs performed by sign in are relative URLs to this
base URL. Previously we ignored this and did it relative to the
current URL, causing the XHRs to 404 and sign in to fall apart.

I presume they do this because you can access the sign in page
from multiple endpoints, such as `/ServiceLogin` and
`/o/oauth2/auth/identifier`

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -43,6 +43,7 @@
 #include <LibWeb/HTML/EventNames.h>
 #include <LibWeb/HTML/HTMLAnchorElement.h>
 #include <LibWeb/HTML/HTMLAreaElement.h>
+#include <LibWeb/HTML/HTMLBaseElement.h>
 #include <LibWeb/HTML/HTMLBodyElement.h>
 #include <LibWeb/HTML/HTMLEmbedElement.h>
 #include <LibWeb/HTML/HTMLFormElement.h>
@@ -523,11 +524,49 @@ Vector<CSS::BackgroundLayerData> const* Document::background_layers() const
     return &body_layout_node->background_layers();
 }
 
+RefPtr<HTML::HTMLBaseElement> Document::first_base_element_with_href_in_tree_order() const
+{
+    RefPtr<HTML::HTMLBaseElement> base_element;
+
+    for_each_in_subtree_of_type<HTML::HTMLBaseElement>([&base_element](HTML::HTMLBaseElement const& base_element_in_tree) {
+        if (base_element_in_tree.has_attribute(HTML::AttributeNames::href)) {
+            base_element = base_element_in_tree;
+            return IterationDecision::Break;
+        }
+
+        return IterationDecision::Continue;
+    });
+
+    return base_element;
+}
+
+// https://html.spec.whatwg.org/multipage/urls-and-fetching.html#fallback-base-url
+AK::URL Document::fallback_base_url() const
+{
+    // FIXME: 1. If document is an iframe srcdoc document, then return the document base URL of document's browsing context's container document.
+    // FIXME: 2. If document's URL is about:blank, and document's browsing context's creator base URL is non-null, then return that creator base URL.
+
+    // 3. Return document's URL.
+    return m_url;
+}
+
+// https://html.spec.whatwg.org/multipage/urls-and-fetching.html#document-base-url
+AK::URL Document::base_url() const
+{
+    // 1. If there is no base element that has an href attribute in the Document, then return the Document's fallback base URL.
+    auto base_element = first_base_element_with_href_in_tree_order();
+    if (!base_element)
+        return fallback_base_url();
+
+    // 2. Otherwise, return the frozen base URL of the first base element in the Document that has an href attribute, in tree order.
+    return base_element->frozen_base_url();
+}
+
 // https://html.spec.whatwg.org/multipage/urls-and-fetching.html#parse-a-url
 AK::URL Document::parse_url(String const& url) const
 {
-    // FIXME: Make sure we do this according to spec.
-    return m_url.complete_url(url);
+    // FIXME: Pass in document's character encoding.
+    return base_url().complete_url(url);
 }
 
 void Document::set_needs_layout()

Userland/Libraries/LibWeb/DOM/Document.h

@@ -66,6 +66,10 @@ public:
 
     void set_url(const AK::URL& url) { m_url = url; }
     AK::URL url() const { return m_url; }
+    AK::URL fallback_base_url() const;
+    AK::URL base_url() const;
+
+    RefPtr<HTML::HTMLBaseElement> first_base_element_with_href_in_tree_order() const;
 
     String url_string() const { return m_url.to_string(); }
     String document_uri() const { return m_url.to_string(); }

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -81,8 +81,8 @@ Node::~Node()
 // https://dom.spec.whatwg.org/#dom-node-baseuri
 String Node::base_uri() const
 {
-    // FIXME: Return this’s node document’s document base URL, serialized.
-    return document().url_string();
+    // Return this’s node document’s document base URL, serialized.
+    return document().base_url().to_string();
 }
 
 const HTML::HTMLAnchorElement* Node::enclosing_link_element() const

Userland/Libraries/LibWeb/HTML/HTMLBaseElement.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/DOM/Document.h>
 #include <LibWeb/HTML/HTMLBaseElement.h>
 
 namespace Web::HTML {
@@ -14,4 +15,52 @@ HTMLBaseElement::HTMLBaseElement(DOM::Document& document, DOM::QualifiedName qua
 }
 
 HTMLBaseElement::~HTMLBaseElement() = default;
+
+void HTMLBaseElement::inserted()
+{
+    HTMLElement::inserted();
+
+    // The frozen base URL must be immediately set for an element whenever any of the following situations occur:
+    // - The base element becomes the first base element in tree order with an href content attribute in its Document.
+
+    // NOTE: inserted() is called after this element has been inserted into the document.
+    auto first_base_element_with_href_in_document = document().first_base_element_with_href_in_tree_order();
+    if (first_base_element_with_href_in_document == this)
+        set_the_frozen_base_url();
+}
+
+void HTMLBaseElement::parse_attribute(FlyString const& name, String const& value)
+{
+    HTMLElement::parse_attribute(name, value);
+
+    // The frozen base URL must be immediately set for an element whenever any of the following situations occur:
+    // - The base element is the first base element in tree order with an href content attribute in its Document, and its href content attribute is changed.
+    if (name != AttributeNames::href)
+        return;
+
+    auto first_base_element_with_href_in_document = document().first_base_element_with_href_in_tree_order();
+    if (first_base_element_with_href_in_document == this)
+        set_the_frozen_base_url();
+}
+
+// https://html.spec.whatwg.org/multipage/semantics.html#set-the-frozen-base-url
+void HTMLBaseElement::set_the_frozen_base_url()
+{
+    // 1. Let document be element's node document.
+    auto& document = this->document();
+
+    // 2. Let urlRecord be the result of parsing the value of element's href content attribute with document's fallback base URL, and document's character encoding. (Thus, the base element isn't affected by itself.)
+    auto href = attribute(AttributeNames::href);
+    auto url_record = document.fallback_base_url().complete_url(href);
+
+    // 3. Set element's frozen base URL to document's fallback base URL, if urlRecord is failure or running Is base allowed for Document? on the resulting URL record and document returns "Blocked", and to urlRecord otherwise.
+    // FIXME: Apply "Is base allowed for Document?" CSP
+    if (!url_record.is_valid()) {
+        m_frozen_base_url = document.fallback_base_url();
+        return;
+    }
+
+    m_frozen_base_url = move(url_record);
+}
+
 }

Userland/Libraries/LibWeb/HTML/HTMLBaseElement.h

@@ -16,6 +16,18 @@ public:
 
     HTMLBaseElement(DOM::Document&, DOM::QualifiedName);
     virtual ~HTMLBaseElement() override;
+
+    AK::URL const& frozen_base_url() const { return m_frozen_base_url; }
+
+    virtual void inserted() override;
+    virtual void parse_attribute(FlyString const& name, String const& value) override;
+
+private:
+    // https://html.spec.whatwg.org/multipage/semantics.html#frozen-base-url
+    // A base element that is the first base element with an href content attribute in a document tree has a frozen base URL.
+    AK::URL m_frozen_base_url;
+
+    void set_the_frozen_base_url();
 };
 
 }

Userland/Libraries/LibWeb/HTML/Scripting/WindowEnvironmentSettingsObject.cpp

@@ -62,9 +62,8 @@ String WindowEnvironmentSettingsObject::api_url_character_encoding()
 // https://html.spec.whatwg.org/multipage/window-object.html#script-settings-for-window-objects:api-base-url
 AK::URL WindowEnvironmentSettingsObject::api_base_url()
 {
-    // FIXME: Return the current base URL of window's associated Document.
-    //        (This currently just returns the current document URL, not accounting for <base> elements and such)
-    return m_window->associated_document().url();
+    // Return the current base URL of window's associated Document.
+    return m_window->associated_document().base_url();
 }
 
 // https://html.spec.whatwg.org/multipage/window-object.html#script-settings-for-window-objects:concept-settings-object-origin

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp

@@ -493,7 +493,7 @@ DOM::ExceptionOr<void> XMLHttpRequest::open(String const& method, String const&
     auto normalized_method = normalize_method(method);
 
     // 6. Let parsedURL be the result of parsing url with settingsObject’s API base URL and settingsObject’s API URL character encoding.
-    auto parsed_url = settings_object.responsible_document()->parse_url(url);
+    auto parsed_url = settings_object.api_base_url().complete_url(url);
 
     // 7. If parsedURL is failure, then throw a "SyntaxError" DOMException.
     if (!parsed_url.is_valid())