Can a html document and a css document have the same filename?

Sure. Why not?

In what way? Do you mean www.example.com/foo and www.example.com/foo ? Or do you mean www.example.com/foo.html and www.example.com/foo.css ?

One time I had trouble getting the external stylesheet to load and I could have sworn it was because both documents had the same filename.

Hey. . . why is it that if I check Enable email notification of replies, rarely do I get the notifications?

I mean www.example.com/foo.html and www.example.com/foo.css

Go for it. There shouldn't be any problems with that.

I think there is a possibility for unexpected results if you are using Apache's content negotiation, URL re-write rules or certain mime type declarations.