This site is otherwise clean XHTML Transitional, but I chose to adopt some Apple-specific extensions to give my Mac users a nicer final product. (These would for enhancing the search bar to your right.)

Extensions are fine as long as they degrade gracefully (as these do), and there should be some kind of system for defining extensions that, while not standard, can be handled elegantly by most standards-compliant browsers.

That being said, if there is a protocol for defining extensions and those are ignored, that ain’t so cool.