At GOV.UK we avoid disabling buttons.
If the user submits a form with a problem, they get a useful error message telling them how to fix it.
Disabled buttons are hard to make accessible for people with low vision, and can't easily be focused using the keyboard
@joelanman With all my respect, I disagree: If a button is disabled, I know for 100% that something is wrong in my form (most of the times it's the wrong phone number format or a forgotten obligatory checkbox). I'm totally blind.
@menelion thanks, however it is a problem for many other people, so its simplest to give an error message telling them how to fix it. Out of interest how do you focus the disabled button?
@joelanman I don't focus it with Tab, of course (it's not focusable, as you pointed out). I either navigate with arrows, or press the B key (works in both JAWS and NVDA on Windows).
@menelion good to know thanks!