As usual a refresh fixes these kind of errors, but it's a bug nontheless.

For what it's worth, I catched the following headers on that request:

Request method: GET
Status code: 304 Not Modified

Cache-Control: public, max-age=300, s-maxage=300
Connection: keep-alive
Date: Fri, 06 May 2011 18:14:55 GMT
Expires: Fri, 06 May 2011 18:15:14 GMT
Last-Modified: Wed, 04 May 2011 20:16:15 GMT
Vary: Accept-Encoding
Via: 1.1 varnish
X-Varnish: 2004502580

Could this be related to bug #28714?

See also bug 28840 and bug 28851 !
See also problem reports at http://meta.wikimedia.org/wiki/Wikimedia_Forum#Troubles

(In reply to comment #3)

See also bug 28840

Why do you think that one is related? From what I can see, that one is an IE issue and what this bug describes it is not IE-specific.

(In reply to comment #4)

Why do you think that one is related?

Blind guess while driven to distraction. I know nothing about how the Resource loader works, but there were a lot of recent bug reports describing failure to render CSS. The symptoms were driving me crazy, crazy enough to hope there might be a single cause. The temporary patch at bug 28840 has relieved my pain, so I will stop groping in the dark.

(In reply to comment #5)

The symptoms were driving me
crazy, crazy enough to hope there might be a single cause. The temporary patch
at bug 28840 has relieved my pain, so I will stop groping in the dark.

We're very interested in tracking this down but we're having trouble reproducing it. Could you let us know what browser you're using? Also, I assume you're seeing this on Wiki* site, not you're own installation of Mediawiki, is that correct?

If you were seeing this pretty consistently, your information would help us a lot.

(In reply to comment #6)

The above linked (in comment 3) thread at Meta describes what I was seeing. I am using IE7 on Vista. Before the temporary patch at bug 28840, I was seeing the problem on "dif" pages 100% consistently at en.wikiquote. (It drove me crazy because I routinely spend 2-3 hours per day reviewing diffs at "Special:Recent changes".) This and other symptoms were only intermittent at Meta, and I am not sure whether I ever saw it at en.wikipedia.

Since the patch (Oooh, the cooolooouuurs!) I have only seen the failure to render CSS three times, and refreshing the page did the trick each time. Those three (at Meta, if I recall, but one may have been at either Meta or en.wikipedia) were a content page, a diff, and Special:Recent changes itself. Clearly there is more than one thing going on. The presumed cause mentioned at http://en.wikipedia.org/wiki/Wikipedia:Wikipedia_Signpost/2011-05-09/Technology_report#In_brief does not seem to correspond to what was done in the patch.

All the bugs mentioned in the previous comments are fixed. I can no longer reproduce this, marking WORKSFORME for now.