LibWeb: Start implementing render-blocking mechanism from HTML spec

This change implements enough spec steps to block rendering until
execution of sync scripts inserted from HTML parser is complete.

Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp

@@ -270,7 +270,10 @@ void EventLoop::update_the_rendering()
         if (!document.is_fully_active())
             return false;
 
-        // FIXME: doc is render-blocked;
+        // doc is render-blocked;
+        if (document.is_render_blocked()) {
+            return false;
+        }
 
         // doc's visibility state is "hidden";
         if (document.hidden())

Libraries/LibWeb/HTML/HTMLElement.cpp

@@ -71,6 +71,37 @@ void HTMLElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_popover_close_watcher);
 }
 
+// https://html.spec.whatwg.org/multipage/dom.html#block-rendering
+void HTMLElement::block_rendering()
+{
+    // 1. Let document be el's node document.
+    auto& document = this->document();
+
+    // 2. If document allows adding render-blocking elements, then append el to document's render-blocking element set.
+    if (document.allows_adding_render_blocking_elements()) {
+        document.add_render_blocking_element(*this);
+    }
+}
+
+// https://html.spec.whatwg.org/multipage/dom.html#unblock-rendering
+void HTMLElement::unblock_rendering()
+{
+    // 1. Let document be el's node document.
+    auto& document = this->document();
+
+    // 2. Remove el from document's render-blocking element set.
+    document.remove_render_blocking_element(*this);
+}
+
+// https://html.spec.whatwg.org/multipage/urls-and-fetching.html#potentially-render-blocking
+bool HTMLElement::is_potentially_render_blocking()
+{
+    // An element is potentially render-blocking if
+    // FIXME: its blocking tokens set contains "render",
+    // or if it is implicitly potentially render-blocking, which will be defined at the individual elements.
+    return is_implicitly_potentially_render_blocking();
+}
+
 // https://html.spec.whatwg.org/multipage/dom.html#dom-dir
 StringView HTMLElement::dir() const
 {

Libraries/LibWeb/HTML/HTMLElement.h

@@ -151,6 +151,15 @@ protected:
 
     virtual void visit_edges(Cell::Visitor&) override;
 
+    // https://html.spec.whatwg.org/multipage/dom.html#block-rendering
+    void block_rendering();
+    // https://html.spec.whatwg.org/multipage/dom.html#unblock-rendering
+    void unblock_rendering();
+    // https://html.spec.whatwg.org/multipage/urls-and-fetching.html#potentially-render-blocking
+    bool is_potentially_render_blocking();
+    // https://html.spec.whatwg.org/multipage/urls-and-fetching.html#implicitly-potentially-render-blocking
+    virtual bool is_implicitly_potentially_render_blocking() const { return false; }
+
 private:
     virtual bool is_html_element() const final { return true; }
 

Libraries/LibWeb/HTML/HTMLScriptElement.cpp

@@ -97,7 +97,8 @@ void HTMLScriptElement::execute_script()
         return;
     }
 
-    // FIXME: 3. Unblock rendering on el.
+    // 3. Unblock rendering on el.
+    unblock_rendering();
 
     // 3. If el's result is null, then fire an event named error at el, and return.
     if (m_result.has<ResultState::Null>()) {
@@ -393,7 +394,10 @@ void HTMLScriptElement::prepare_script()
             return;
         }
 
-        // FIXME: 7. If el is potentially render-blocking, then block rendering on el.
+        // 7. If el is potentially render-blocking, then block rendering on el.
+        if (is_potentially_render_blocking()) {
+            block_rendering();
+        }
 
         // 8. Set el's delaying the load event to true.
         begin_delaying_document_load_event(*m_preparation_time_document);
@@ -664,4 +668,11 @@ WebIDL::ExceptionOr<void> HTMLScriptElement::cloned(Node& copy, bool subtree) co
     return {};
 }
 
+// https://html.spec.whatwg.org/multipage/urls-and-fetching.html#implicitly-potentially-render-blocking
+bool HTMLScriptElement::is_implicitly_potentially_render_blocking() const
+{
+    // A script element el is implicitly potentially render-blocking if el's type is "classic", el is parser-inserted, and el does not have an async or defer attribute.
+    return m_script_type == ScriptType::Classic && is_parser_inserted() && !has_attribute(AttributeNames::async) && !has_attribute(AttributeNames::defer);
+}
+
 }

Libraries/LibWeb/HTML/HTMLScriptElement.h

@@ -65,6 +65,10 @@ public:
 
     virtual WebIDL::ExceptionOr<void> cloned(Node&, bool) const override;
 
+protected:
+    // https://html.spec.whatwg.org/multipage/urls-and-fetching.html#implicitly-potentially-render-blocking
+    virtual bool is_implicitly_potentially_render_blocking() const override;
+
 private:
     HTMLScriptElement(DOM::Document&, DOM::QualifiedName);