It's still only a dummy as LibWeb doesn't have focused elements yet, but at least now we don't treat "selector:focus" as just "selector". This fixes an issue on google.com which was mostly grey - coming from some menu item focus styles :^)
This makes selector matching a lot more efficient, and also reduces the number of strings on the heap.
Let's rename this to LibWeb since it aims to provide more parts of the web platform than just HTML. :^)
