For pages already specified (either by HTTP header, or by meta tag), to have a Content-Type with a UTF-8 charset... is there a benefit of adding accept-charset="UTF-8" to HTML forms?
(I understand the accept-charset attribute is broken in IE for ISO-8859-1, but I haven't heard of a problem with IE and UTF-8. I'm just asking if there's a benefit to adding it with UTF-8, to help prevent invalid byte sequences from being entered.)
I did not encounter any problems using UTF-8 with IE (6+) or any other major browser out there. You need to make sure, that a UTF-8 meta tag is set (IE needs this) and that all your files are UTF-8 encoded (which means that the webserver sends UTF-8 headers). Then there should not be any problem if you omit accept-charset.
If the page is already interpreted by the browser as being UTF-8, setting accept-charset="utf-8" does nothing.
If you set the encoding of the page to UTF-8 in a <meta> and/or HTTP header, it will be interpreted as UTF-8, unless the user deliberately goes to the View->Encoding menu and selects a different encoding, overriding the one you specified.
In that case, accept-encoding would have the effect of setting the submission encoding back to UTF-8 in the face of the user messing about with the page encoding. However, this still won't work in IE, due the previous problems discussed with accept-encoding in that browser.
So it's IMO doubtful whether it's worth including accept-charset to fix the case where a non-IE user has deliberately sabotaged the page encoding (possibly messing up more on your page than just the form). Personally, I don't bother.