I'm tempted to dismiss it as "works for me," but I can't quite get past the fact that the reported cases all seem to be fixed by restarting Sigil. If it was a syntax error or other invalid css, a restart/reopen shouldn't have "fixed" it. A restart of Sigil should never make anything render differently, should it (provided there were no unsaved changes dismissed)? Are there any circumstances where a file rename/import or a split/merge could somehow leave something "stale" or disconnected? Those would be the areas I would focus on if I were looking to nail down something reproduceable. That or any automated task that was run prior to css changes not taking effect any more.
One other possibility is embedded fonts. I'd have no trouble believing there might be issues with css changes not immediately taking effect on text that was already being rendered with embedded fonts. Embedded fonts and WebKit have been pretty flaky from the beginning.
Help us find it and we'll fix it (or at least tell you we
can't)!
