There are [[ https://de.wikipedia.org/wiki/Wikipedia:Fragen_zur_Wikipedia#CSS_wird_nicht_geladen | 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.
**=> Upstream: <https://bugzilla.mozilla.org/show_bug.cgi?id=712130>**
Summary:
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>