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It's difficult to say why you're getting this error without more detail. You might be able to get more information from a different validator, http://validator.w3.org/#validate_by_upload.
Give this a try and see if it gives you more information about what the problem is.
Also, make sure you look below the main message you get, usually there will be details there. When I tried validating a txt document (so I knew it would be invalid), here's what I got:
The details of why this document is not valid: "Could not validate this document, because it does not include a DOCTYPE Declaration."
Here are three things I would check:
- Is the file you're trying to validate a .html file?
- Do you have a proper DOCTYPE declaration?
- Are the main tags on the page opened and closed properly? (<html> <head> </head> <body> </body> </html>)
I hope this helps. If we can figure out a few details, then we can help you pin down the problem.
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