LibWeb: Add loading event to style element

Author: https://github.com/lpas
Commit: 7260159b8f
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6479

Libraries/LibWeb/DOM/StyleElementUtils.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2023, Preston Taylor <PrestonLeeTaylor@proton.me>
+ * Copyright (c) 2025, Lorenz Ackermann <me@lorenzackermann.xyz>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -8,6 +9,7 @@
 #include <LibWeb/CSS/StyleComputer.h>
 #include <LibWeb/ContentSecurityPolicy/BlockingAlgorithms.h>
 #include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/Event.h>
 #include <LibWeb/DOM/ShadowRoot.h>
 #include <LibWeb/DOM/StyleElementUtils.h>
 #include <LibWeb/Infra/Strings.h>
@@ -97,6 +99,38 @@ void StyleElementUtils::update_a_style_block(DOM::Element& style_element)
         style_element.document().script_blocking_style_sheet_set().set(style_element);
 
     // FIXME: 8. If element's media attribute's value matches the environment and element is potentially render-blocking, then block rendering on element.
+
+    // https://html.spec.whatwg.org/multipage/semantics.html#the-style-element:critical-subresources
+    auto attempts_to_fetch_subresources_finished = [](GC::Ptr<CSS::CSSStyleSheet> style_sheet) {
+        // 1. Let element be the style element associated with the style sheet in question.
+        auto& element = *style_sheet->owner_node();
+        // 2. Let success be true.
+        auto success = true;
+        // FIXME: 3. If the attempts to obtain any of the style sheet's critical subresources failed for any reason
+        // (e.g., DNS error, HTTP 404 response, a connection being prematurely closed, unsupported Content-Type), set success to false.
+        // Note: that content-specific errors, e.g., CSS parse errors or PNG decoding errors, do not affect success.
+        // 4. Queue an element task on the networking task source given element and the following steps:
+        element.queue_an_element_task(HTML::Task::Source::UserInteraction, [&element, success] {
+            // 1. If success is true, fire an event named load at element.
+            // AD-HOC: these should call fire an event this is not implemented anywhere so we dispatch ourself
+            if (success)
+                element.dispatch_event(DOM::Event::create(element.realm(), HTML::EventNames::load));
+            // 2. Otherwise, fire an event named error at element.
+            else
+                element.dispatch_event(DOM::Event::create(element.realm(), HTML::EventNames::error));
+            // 3. If element contributes a script-blocking style sheet:
+            if (element.contributes_a_script_blocking_style_sheet()) {
+                // 1. Assert: element's node document's script-blocking style sheet set contains element.
+                VERIFY(element.document().script_blocking_style_sheet_set().contains(element));
+                // 2. Remove element from its node document's script-blocking style sheet set.
+                element.document().script_blocking_style_sheet_set().remove(element);
+            }
+            // 4. Unblock rendering on element.
+            element.unblock_rendering();
+        });
+    };
+    // FIXME: The element must delay the load event of the element's node document until all the attempts to obtain the style sheet's critical subresources, if any, are complete.
+    attempts_to_fetch_subresources_finished(m_associated_css_style_sheet);
 }
 
 void StyleElementUtils::visit_edges(JS::Cell::Visitor& visitor)