Instead of string splitting every time you call Element::has_class(),
we now split the "class" attribute value when it changes, and cache
the individual classes as FlyStrings in Element::m_classes.
This makes has_class() significantly faster and moves the pain point
of selector matching somewhere else.
Getting the innerHTML property will recurse through the subtree inside
the element and serialize it into a string as it goes.
Setting it will parse the set value as an HTML fragment. It will then
remove all current children of the element and replace them with all
the children inside the parsed fragment.
Setting element.innerHTML will currently force a complete rebuild of
the document's layout tree.
This is pretty neat! :^)
