There are reports on de.wikipedia that in Firefox (and perhaps also other browsers, but less frequent) the stylesheets are sometimes loaded too late. This happens both in Firefox 57 and in previous versions.
I can reliably reproduce this on Special:Search (in any WMF wiki, including beta.wmflabs), both logged in and out, with cache and uncached, so this doesn't seem to be some intermittent network issue.
Every time I load or reload https://de.wikipedia.org/w/index.php?search=&title=Spezial%3ASuche&go=Artikel or similar, I briefly see the page rendered without CSS (every time in Firefox ESR; with Firefox 57 it still happens very often, but not every time).
Though according to the reports other pages are affected as well, I can only reproduce on Special:Search, but as said, there it fails reliably.
Pages with an autofocus input field render in Firefox without waiting for stylesheets to apply. This is mainly observed on three special pages:
- Search, e.g. https://de.wikipedia.org/w/index.php?title=Spezial:Suche&search=Artikel
- Log in, e.g. https://en.wikipedia.org/w/index.php?title=Special:UserLogin
- Book sources (ISBN citation), e.g. https://en.wikipedia.org/wiki/Special:Booksources
=> Upstream: https://bugzilla.mozilla.org/show_bug.cgi?id=712130
Update as of 14 February 2018: The bug has been fixed in Firefox Nightly. The solution is expected to be part of Firefox 60, (release notes), which will be Beta starting March 12, and will be released to stable starting May 8. (release schedule)