None of these facts imply that either CSS lookup cannot be sped up by caching or XUL didn't already have those concerns. And for something like the browser chr

None of these facts imply that either CSS lookup cannot be sped up by caching or XUL didn't already have those concerns. And for something like the browser chrome, the cache could even be serialized during compilation and loaded in to prime it when Firefox launches. It's possible to build a slow CSS engine and it's possible to build a fast CSS engine.