jQuery is neither a cross-browser library nor a standard of any kind. Same for the other plugs. This is supposed to be a technical document, not an infomercial for dubious scripts. :)
Why does MDN look different?
MDN is changing to focus just on documenting web technologies. All the same great content is still here; we’re just changing some visual elements and navigation, to help you more quickly find the web technology docs you’re looking for.
But don’t worry, MDN and Mozilla are still together! In fact, we're updating MDN’s look to reflect Mozilla's new logo and colors.