From the ucam-webmasters mailing list — yet another reason to switch to Firefox:
After application of SP2 to Windows XP there seems to be an interaction that we haven’t yet managed to isolate, which causes html files sent from IE to the validator at validator.w3.org (by either method) to be given the mime type of text. This causes validation to fail. To get round this difficulty, we suggest that you use another browser, such as Mozilla/Firefox or Opera. If you are producing web pages, the web developers toolbar for Mozilla/Firefox allows you to validate directly, as well as all sorts of other useful functionality.
The e-mail goes on to say that Mozilla is now available on the Cambridge Public Workstation Facility (PWF) — although my housemate said he couldn’t find it when he looked last week! No mention of firefox though, but they do have Opera which I have heard many people rave about.
12:15pm: Another e-mail on the same list. Apparently the problem only effects file-uploads — submitting the URI into the webform works fine.