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/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
 {