On Mathematics, Arqade, Electrical Engineering, and Ask Different (to list those which I have noticed), the styling on form submit buttons (such as “Post Your Question” and “Save Edits”) removes all cues indicating that the button has the focus. This is a minor usability problem for me — I like to operate forms by keyboard and when I tab past the text fields there is no cue for whether the focus has gone to the button, or some other control — and I am given to understand it is a larger (accessibility) problem for others.
I request that the stylesheets for those sites which have this problem be changed such that there is some focus indication. I don't care whether it is the browser default or something custom.
(Since I originally wrote this question, I note that some more recently launched/designed sites, such as skeptics.SE and scifi.SE, don't have this problem. Good! But the old instances are still around.)