Miles Bader <miles@gnu.org> writes:

> It looks reasonable in your picture, but remember that the "button"
> attribute is also commonly used for mode-lines.

The default mode-line has width 1, so it's unaffected.  Setting the
width to 2, the result still also looks reasonable; see attached.

> Also, how about on black backgrounds?  It will just make the relief look
> "thinner", right?

Seems fine to me (I use a black background myself).  The button borders
legibility issue isn't generally a problem for black backgrounds, when
the button itself is grey.