The patch highlights the currently active input element
using CSS (:focus).

For input elements of type "submit" or "button" it
tries to suppress the background change, but AFAIK, and
at the time of this writing, only Opera (v9.01) does
this right; SeaMonkey 1.0.4 (Gecko/20060729) can by no
means be convinced to use the inherited background (it
/would/ use any other value, including transparent, but
this doesn't look right either). The usage of
<button...> tags would avoid this problem.

No browser known to me changes the background of <input
type=file...> tags. MSIE 6.0 does nothing at all.

However, for the users of Opera, any flavour of Mozilla
etc. (and likely) the feature /is/ useful, especially
when the focus has been set programmatically (e.g.
because of missing fields, like it is done by the
checkRequiredFields function of sf.net issue 1539081).