Back in July, we had a lengthy discussion about the possibility of switching to HTML5:
http://lists.wikimedia.org/pipermail/wikitech-l/2009-July/043865.html
The conclusion appeared to be that switching to HTML5 was a good idea. As of July 12 (r53142), MediaWiki has been outputting HTML5 continuously as the default:
http://www.mediawiki.org/wiki/Special:Code/MediaWiki/53142
There have been only a few minor problems reported by various trunk users, all of which were quickly fixed. The current state of HTML5 support is discussed here:
http://www.mediawiki.org/wiki/HTML5
When we had our last scap six weeks ago, $wgHtml5 was disabled on Wikimedia to reduce the number of variables to keep track of, since a huge amount of code was changing at once. Although we're currently in the middle of some organizational issues due to Brion's resignation, I think it would be safe to experimentally turn on $wgHtml5 and see what happens. If any problems are reported that can't be fixed immediately, of course, it could be turned off again, but I think this is quite unlikely. I'll be available for the next few weeks to work on any issues that arise. Would this be okay?