Using the CSS validator on a CSS file process it but fails to open a browser window as the HTML validator on the validator site. Instead the validator pop up window remains and the whole program hangs, the only way out is to kill the process. Happen when other html pages are open.
I understand this as you are trying to validate a css file using the 'Validate css' found under the 'Document' menu, and the process doesn't open a browser window. Which of them hang, the html Editor or the browser?
I have tried having my html extension associated with my default browser (Ffx) and with the Editor itself. In the former case the browser opens without problem, in the latter case the Editor opens the code window, also without problems, and the page can be previewed just like any other page. I have two or three html pages open and one css file. No problem at this end with either, and I don't recall having had such problems earlier. The css file I tested this with is not particularly complex, but it validated. Is your file possibly very long, very complex and has a lot of errors?