LibWeb: Implement more spec-compliant iframe/frame attribute processing

We're still missing the lazy loading attribute handling, and once we hit
the navigation step, we fall back to totally ad-hoc behavior instead of
going all the way with a Fetch Request.

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp

@@ -46,10 +46,51 @@ void HTMLIFrameElement::inserted()
         // FIXME: 2. If element has a sandbox attribute, then parse the sandboxing directive given the attribute's value and element's iframe sandboxing flag set.
 
         // 3. Process the iframe attributes for element, with initialInsertion set to true.
+        process_the_iframe_attributes(true);
         load_src(attribute(HTML::AttributeNames::src));
     }
 }
 
+// https://html.spec.whatwg.org/multipage/urls-and-fetching.html#will-lazy-load-element-steps
+bool HTMLIFrameElement::will_lazy_load_element() const
+{
+    // 1. If scripting is disabled for element, then return false.
+    if (document().is_scripting_disabled())
+        return false;
+
+    // FIXME: 2. If element's lazy loading attribute is in the Lazy state, then return true.
+
+    // 3. Return false.
+    return false;
+}
+
+// https://html.spec.whatwg.org/multipage/iframe-embed-object.html#process-the-iframe-attributes
+void HTMLIFrameElement::process_the_iframe_attributes(bool initial_insertion)
+{
+    // 1. If element's srcdoc attribute is specified, then:
+    if (has_attribute(HTML::AttributeNames::srcdoc)) {
+        // 2. Set element's current navigation was lazy loaded boolean to false.
+        m_current_navigation_was_lazy_loaded = false;
+
+        // 3. If the will lazy load element steps given element return true, then:
+        if (will_lazy_load_element()) {
+            // FIXME: 1. Set element's lazy load resumption steps to the rest of this algorithm starting with the step labeled navigate to the srcdoc resource.
+            // FIXME: 2. Set element's current navigation was lazy loaded boolean to true.
+            // FIXME: 3. Start intersection-observing a lazy loading element for element.
+            // FIXME: 4. Return.
+        }
+
+        // FIXME: 4. Navigate to the srcdoc resource: navigate an iframe or frame given element and a new response whose URL list is « about:srcdoc », header list is « (`Content-Type`, `text/html`) », and body is the value of element's srcdoc attribute.
+
+        // FIXME: The resulting Document must be considered an iframe srcdoc document.
+
+        return;
+    }
+
+    // 2. Otherwise, run the shared attribute processing steps for iframe and frame elements given element and initialInsertion.
+    shared_attribute_processing_steps_for_iframe_and_frame(initial_insertion);
+}
+
 // https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-iframe-element:the-iframe-element-7
 void HTMLIFrameElement::removed_from(DOM::Node* node)
 {