FBML
That actually stands for “Facebook Markup Language”
But it’s fairly clear what the real acronym should be.

FBML? You know what the acronym should really be
Turns out that specific HTML pages are no longer allowed for your Canvas URL in Facebook Apps and Pages. You need to now link to either a dynamic page, or a site directory.
You may receive the following error:
Validation failed. URL must point to a directory (i.e., end with a ‘/’ or a dynamic page (i.e., have a ‘?’ somewhere).
At least that error used to at least indicate that it was the Canvas URL that was the issue…It’s now just a lot more vague.
FML… I mean, FBML
You may have been using:
http://dwayneali.com/thisiswhereisstorestuff/facebooksomething.html
but now you’ll only be allowed:
http://dwayneali.com/thisiswhereistorestuff/
Quick fix? Depending on your server setup (ie, Apache) you may just be able to rename your html file to ‘index.html’ and then just refer to the directory. Apache will serve that file first….and Facebook will let you get away with the directory name.