LibWeb: Implement more of the "script-blocking style sheet" mechanism

The basic idea is that style sheets can block script execution under
some circumstances. With this commit, we now handle the simplest cases
where a parser-inserted link element gets to download its style sheet
before script execution continues.

This improves performance on Speedometer 3 where JavaScript APIs that
depend on layout results (like Element.scrollIntoView()) would get
called too early (before the relevant CSS was downloaded), and so we'd
perform premature layout work. This work then had to be redone after
downloading the CSS anyway, wasting time.

Note that our Text/input/link-re-enable-crash.html test had to be
tweaked after these changes, since it relied on the old, incorrect,
behavior where scripts would run before downloading CSS.

Libraries/LibWeb/HTML/HTMLLinkElement.cpp

@@ -509,9 +509,14 @@ void HTMLLinkElement::process_stylesheet_resource(bool success, Fetch::Infrastru
         dispatch_event(*DOM::Event::create(realm(), HTML::EventNames::error));
     }
 
-    // FIXME: 6. If el contributes a script-blocking style sheet, then:
-    //     FIXME: 1. Assert: el's node document's script-blocking style sheet counter is greater than 0.
-    //     FIXME: 2. Decrement el's node document's script-blocking style sheet counter by 1.
+    // 6. If el contributes a script-blocking style sheet, then:
+    if (contributes_a_script_blocking_style_sheet()) {
+        // 1. Assert: el's node document's script-blocking style sheet set contains el.
+        VERIFY(document().script_blocking_style_sheet_set().contains(*this));
+
+        // 2. Remove el from its node document's script-blocking style sheet set.
+        document().script_blocking_style_sheet_set().remove(*this);
+    }
 
     // 7. Unblock rendering on el.
     unblock_rendering();
@@ -541,7 +546,10 @@ bool HTMLLinkElement::stylesheet_linked_resource_fetch_setup_steps(Fetch::Infras
     // 1. If el's disabled attribute is set, then return false.
     if (has_attribute(AttributeNames::disabled))
         return false;
-    // FIXME: 2. If el contributes a script-blocking style sheet, increment el's node document's script-blocking style sheet counter by 1.
+
+    // 2. If el contributes a script-blocking style sheet, append el to its node document's script-blocking style sheet set.
+    if (contributes_a_script_blocking_style_sheet())
+        document().script_blocking_style_sheet_set().set(*this);
 
     // 3. If el's media attribute's value matches the environment and el is potentially render-blocking, then block rendering on el.
     // FIXME: Check media attribute value.
@@ -674,4 +682,34 @@ void HTMLLinkElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_sizes);
 }
 
+// https://html.spec.whatwg.org/multipage/semantics.html#contributes-a-script-blocking-style-sheet
+bool HTMLLinkElement::contributes_a_script_blocking_style_sheet() const
+{
+    // An element el in the context of a Document of an HTML parser or XML parser
+    // contributes a script-blocking style sheet if all of the following are true:
+
+    // el was created by that Document's parser.
+    if (m_parser_document != &document())
+        return false;
+
+    // FIXME: el is either a style element or a link element that was an external resource link that contributes to the styling processing model when the el was created by the parser.
+
+    // FIXME: el's media attribute's value matches the environment.
+
+    // FIXME: el's style sheet was enabled when the element was created by the parser.
+    if (has_attribute(AttributeNames::disabled))
+        return false;
+
+    // FIXME: The last time the event loop reached step 1, el's root was that Document.
+
+    // The user agent hasn't given up on loading that particular style sheet yet.
+    // A user agent may give up on loading a style sheet at any time.
+    if (m_fetch_controller && m_fetch_controller->state() == Fetch::Infrastructure::FetchController::State::Terminated)
+        return false;
+    if (m_fetch_controller && m_fetch_controller->state() == Fetch::Infrastructure::FetchController::State::Aborted)
+        return false;
+
+    return true;
+}
+
 }