Last modified: 2011-11-29 19:57:33 UTC
Per http://www.w3.org/TR/html4/present/graphics.html#h-15.2.1 it's defined that <s> and <u> is deprecated, so to ensure standard compabillity, they should be removed, or redefined using css.
Deprecated is not the same as prohibited. There remains a use for nonsemantic markup. There's no particularly good reason for them to remain in the interface, by and large, but banning the tags from articles isn't useful. It will just mean that people will use <del> and <ins> as synonyms of those two, which they aren't. Accessibility to the non-technical (who generally understand presentational markup much more easily than semantic) is more important than theoretical standards adherence. There are only a handful of places where these are used in the interface, though, so that should be easily fixable.
If at some future time we migrate MediaWiki to a version of HTML in which <u> and <s> are dropped, they will be changed to output something silly like <span style='text-decoration: underline'> etc. There is no reason to do this now.
*** This bug has been marked as a duplicate of bug 4557 ***
