LibWeb: Implement getComputedStyle() pseudoElement parameter

Right now, we deviate from the CSSOM spec regarding our
CSSStyleDeclaration classes, so this is not as close to the spec as I'd
like. But it works, which means we'll be able to test pseudo-element
styling a lot more easily. :^)

Userland/Libraries/LibWeb/HTML/Window.cpp

@@ -1136,9 +1136,51 @@ Variant<JS::Handle<DOM::Event>, JS::Value> Window::event() const
 // https://w3c.github.io/csswg-drafts/cssom/#dom-window-getcomputedstyle
 JS::NonnullGCPtr<CSS::CSSStyleDeclaration> Window::get_computed_style(DOM::Element& element, Optional<String> const& pseudo_element) const
 {
-    // FIXME: Make this fully spec compliant.
-    (void)pseudo_element;
-    return heap().allocate<CSS::ResolvedCSSStyleDeclaration>(realm(), element);
+    // 1. Let doc be elt’s node document.
+
+    // 2. Let obj be elt.
+    Optional<CSS::Selector::PseudoElement::Type> obj_pseudo;
+
+    // 3. If pseudoElt is provided, is not the empty string, and starts with a colon, then:
+    if (pseudo_element.has_value() && pseudo_element.value().starts_with(':')) {
+        // 1. Parse pseudoElt as a <pseudo-element-selector>, and let type be the result.
+        auto type = parse_pseudo_element_selector(CSS::Parser::ParsingContext(associated_document()), pseudo_element.value());
+
+        // 2. If type is failure, or is an ::slotted() or ::part() pseudo-element, let obj be null.
+        // FIXME: We can't pass a null element to ResolvedCSSStyleDeclaration
+        if (!type.has_value()) {
+        }
+        // 3. Otherwise let obj be the given pseudo-element of elt.
+        else {
+            // TODO: Keep the function arguments of the pseudo-element if there are any.
+            obj_pseudo = type.value().type();
+        }
+    }
+
+    // AD-HOC: Just return a ResolvedCSSStyleDeclaration because that's what we have for now.
+    // FIXME: Implement CSSStyleProperties, and then follow the rest of these steps instead.
+    return heap().allocate<CSS::ResolvedCSSStyleDeclaration>(realm(), element, obj_pseudo);
+
+    // 4. Let decls be an empty list of CSS declarations.
+
+    // 5. If obj is not null, and elt is connected, part of the flat tree, and its shadow-including root
+    //    has a browsing context which either doesn’t have a browsing context container, or whose browsing
+    //    context container is being rendered, set decls to a list of all longhand properties that are
+    //    supported CSS properties, in lexicographical order, with the value being the resolved value
+    //    computed for obj using the style rules associated with doc. Additionally, append to decls all
+    //    the custom properties whose computed value for obj is not the guaranteed-invalid value.
+
+    // 6. Return a live CSSStyleProperties object with the following properties:
+    //    computed flag
+    //        Set.
+    //    readonly flag
+    //        Set.
+    //    declarations
+    //        decls.
+    //    parent CSS rule
+    //        Null.
+    //    owner node
+    //        obj.
 }
 
 // https://w3c.github.io/csswg-drafts/cssom-view/#dom-window-matchmedia