[CSS21] text-decoration on replaced elements that "contain text"

What's the correct behavior of "text-decoration:underline" on something like a 
<input type="button" value="This is text">?  CSS2.1 says:

"If an element contains no text (ignoring white space in elements that have
'white-space' set to 'normal', 'pre-line', or 'no-wrap'), user agents must
refrain from rendering text decorations on the element."

which seems to be somewhat open to interpretation as to what "contains" means. 
See https://bugzilla.mozilla.org/show_bug.cgi?id=219516#c15 for example.

-Boris