I changed my theme which defined styles on the body element. The changes were not reflected inside the web content editor textarea. Firebug shows an iframe inside of which the theme css is linked without its querystring, such as:
good link in top level html document:
bad link within nested document:
When I put the above css address without querystring into the browser address bar and clicked reload, the browser reloaded the css correctly and now as well shows the correct style inside the editor.
Honestly, I have no clue where that querystring might have gone. In order to reproduce, it should be sufficient to check if there is a querystring present inside the editor iframe css for any theme.