When external stylesheet specifies that a block floats and the top margin of a block below the floated block, the floated object is layout incorrectly. When the external stylesheet is cached or declared within the same document, the problem doesn't reproduce.
Created attachment 90596 [details] demo (requires theme.css)
Created attachment 90597 [details] theme.css
To reproduce the bug, download the attached files and put them in a same directory, and open floating-bug.html. This bug only reproduces on Apple's Mac and Chromium Mac ports.