LibWeb: Skip transitions for pseudo elements

Transitions are currently not implemented for pseudo elements which
causes the transition to be applied to the "real"/"parent" element. When
a transition adjusts width/height on a pseudo element this causes the
real elements layout to break.

As a quick fix we just skip doing transitions when they are against
pseudo elements.

(cherry picked from commit 30f59cfe1aec0b385631a40da8b2447eed62ad79)

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -1204,7 +1204,8 @@ static void compute_transitioned_properties(StyleProperties const& style, DOM::E
 void StyleComputer::start_needed_transitions(StyleProperties const& previous_style, StyleProperties& new_style, DOM::Element& element, Optional<Selector::PseudoElement::Type> pseudo_element) const
 {
     // FIXME: Implement transitions for pseudo-elements
-    (void)pseudo_element;
+    if (pseudo_element.has_value())
+        return;
 
     // https://drafts.csswg.org/css-transitions/#transition-combined-duration
     auto combined_duration = [](Animations::Animatable::TransitionAttributes const& transition_attributes) {