script: Make stylesheets loaded via <link> elements block the rendering (#39536)

Stylesheets loaded via the `<link>` element should block the rendering
of the page according to the HTML specification [1]. This change makes
it so that they do this and, in addition, we do not take reftest
screenshots until all no element is blocking the rendering.

This change does not add support for the `blocking` attribute of
`<link>`, but that can be added in a follow change. In addition to
fixing a few tests, this change likely makes other tests no longer
intermittent. We will need to watch CI runs after this lands in order to
verify that though.

Testing: This change fixes at least two WPT tests.
Fixes: #26424.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/document.rs

@@ -350,6 +350,9 @@ pub(crate) struct Document {
     pending_parsing_blocking_script: DomRefCell<Option<PendingScript>>,
     /// Number of stylesheets that block executing the next parser-inserted script
     script_blocking_stylesheets_count: Cell<u32>,
+    /// Number of elements that block the rendering of the page.
+    /// <https://html.spec.whatwg.org/multipage/#implicitly-potentially-render-blocking>
+    render_blocking_element_count: Cell<u32>,
     /// <https://html.spec.whatwg.org/multipage/#list-of-scripts-that-will-execute-when-the-document-has-finished-parsing>
     deferred_scripts: PendingInOrderScriptVec,
     /// <https://html.spec.whatwg.org/multipage/#list-of-scripts-that-will-execute-in-order-as-soon-as-possible>
@@ -1728,6 +1731,21 @@ impl Document {
         count_cell.set(count_cell.get() - 1);
     }
 
+    pub(crate) fn render_blocking_element_count(&self) -> u32 {
+        self.render_blocking_element_count.get()
+    }
+
+    pub(crate) fn increment_render_blocking_element_count(&self) {
+        let count_cell = &self.render_blocking_element_count;
+        count_cell.set(count_cell.get() + 1);
+    }
+
+    pub(crate) fn decrement_render_blocking_element_count(&self) {
+        let count_cell = &self.render_blocking_element_count;
+        assert!(count_cell.get() > 0);
+        count_cell.set(count_cell.get() - 1);
+    }
+
     pub(crate) fn invalidate_stylesheets(&self) {
         self.stylesheets.borrow_mut().force_dirty(OriginSet::all());
 
@@ -2705,6 +2723,10 @@ impl Document {
     //
     // Returns the set of reflow phases run as a [`ReflowPhasesRun`].
     pub(crate) fn update_the_rendering(&self) -> ReflowPhasesRun {
+        if self.render_blocking_element_count() > 0 {
+            return Default::default();
+        }
+
         if self.has_pending_animated_image_update.get() {
             self.image_animation_manager
                 .borrow()
@@ -3389,7 +3411,8 @@ impl Document {
             has_focus: Cell::new(has_focus),
             current_script: Default::default(),
             pending_parsing_blocking_script: Default::default(),
-            script_blocking_stylesheets_count: Cell::new(0u32),
+            script_blocking_stylesheets_count: Default::default(),
+            render_blocking_element_count: Default::default(),
             deferred_scripts: Default::default(),
             asap_in_order_scripts_list: Default::default(),
             asap_scripts_set: Default::default(),

components/script/dom/window.rs

@@ -2347,6 +2347,10 @@ impl Window {
             return;
         }
 
+        if document.render_blocking_element_count() > 0 {
+            return;
+        }
+
         // Checks if the html element has reftest-wait attribute present.
         // See http://testthewebforward.org/docs/reftests.html
         // and https://web-platform-tests.org/writing-tests/crashtest.html

components/script/stylesheet_loader.rs

@@ -277,6 +277,15 @@ impl FetchResponseListener for StylesheetContext {
             document.decrement_script_blocking_stylesheet_count();
         }
 
+        // From <https://html.spec.whatwg.org/multipage/#link-type-stylesheet>:
+        // > A link element of this type is implicitly potentially render-blocking if the element
+        // > was created by its node document's parser.
+        if matches!(self.source, StylesheetContextSource::LinkElement { .. }) &&
+            owner.parser_inserted()
+        {
+            document.decrement_render_blocking_element_count();
+        }
+
         document.finish_load(LoadType::Stylesheet(self.url.clone()), CanGc::note());
 
         if let Some(any_failed) = owner.load_finished(successful) {
@@ -377,6 +386,15 @@ impl ElementStylesheetLoader<'_> {
             document.increment_script_blocking_stylesheet_count();
         }
 
+        // From <https://html.spec.whatwg.org/multipage/#link-type-stylesheet>:
+        // > A link element of this type is implicitly potentially render-blocking if the element
+        // > was created by its node document's parser.
+        if matches!(context.source, StylesheetContextSource::LinkElement { .. }) &&
+            owner.parser_inserted()
+        {
+            document.increment_render_blocking_element_count();
+        }
+
         // https://html.spec.whatwg.org/multipage/#default-fetch-and-process-the-linked-resource
         let global = self.element.global();
         let request = create_a_potential_cors_request(

tests/wpt/meta/html/dom/render-blocking/parser-inserted-stylesheet-link.html.ini

@@ -1,3 +0,0 @@
-[parser-inserted-stylesheet-link.html]
-  [Rendering is blocked before render-blocking resources are loaded]
-    expected: FAIL

tests/wpt/meta/html/dom/render-blocking/remove-attr-stylesheet-link-keeps-blocking.html.ini

@@ -1,3 +0,0 @@
-[remove-attr-stylesheet-link-keeps-blocking.html]
-  [Rendering is blocked before render-blocking resources are loaded]
-    expected: FAIL