Created attachment 233801 [details] An HTML document that exhibits the described behaviour. If you load up the attached HTML in webkit and resize your browser window repeatedly above and below the 800px boundary, the table cells increasingly fall out of line with where they should be. Chrome has a similar, possibly related behaviour where each crossing of the threshold moves the cells one place to the right. Nothing especially weird happens in Gecko. Can reproduce in current nightly and in current stable Safari (7.0.4)
The analogous Chromium bug: https://code.google.com/p/chromium/issues/detail?id=389129
Fixed in Chrome. The pseudo-elements have nothing to do with it; see the comments for details.
This appears not to be an issue any more in WebKit 12603.1.2.