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Htmlhelp.com offers an excellent XHTML validation service. You can upload documents from your machine or provide a URL. Here is the site.
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http://www.htmlhelp.com/tools/validator/upload.html
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W3C HTML Validation Service. You can use the W3 Consortium's HTML validator to check your web documents either by giving it your web page's URL or uploading your document directly to the validator. Many people consider it the authoritative source for checking your web pages for standards compliance (since the W3 Consortium is responsible for the various HTML/CSS standards). Source code for the validator is also available.
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http://validator.w3.org/
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Tidy Online. This is an online version of the HTML Tidy validator that you can use to clean up your web pages if you're too lazy to download your own copy.
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http://cgi.w3.org/cgi-bin/tidy