When you try to validate an HTML file at http://validator.w3.org/
you receive the following error message:
Sorry, I am unable to validate this document because its content type is text/plain, which is not currently supported by this service.
Microsoft Windows XP Service Pack 2 is installed on your computer.
To resolve this problem, install the MS05-020 security update.
For more information about how to install the MS05-020 security update, click the following article number to view the article in the Microsoft Knowledge Base:
MS05-020: Cumulative security update for Internet Explorer