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Free HTML Validators

W3C HTML Validation Service
Validate by typing in an URL or uploading a file.

Checky
Document validation and analysis Extension for Firefox, Mozilla and Netscape.

HTML Tidy
Dave Raggett's HTML TIDY is a free utility that can clean up code into nicely layed out markup. Tidy is able to fix up a wide range of problems and to bring to your attention things that you need to work on yourself. It is available in binaries and source for many platforms.

Tidy your HTML
This is an online version of Tidy at the W3C.

Web Page Purifier
Allows authors to view a page purified to be compliant with various versions of HTML.

WDG HTML Validator
Enter the URL of an HTML document to validate.

Web Page Backward Compatibility Viewer
Enter an URL and a browser and see what your page looks like in older versions.

Web Accessibility Evaluation Tool (WAVE)
This free service will allow you to test web pages and help expose and repair barriers to accessibility and encourage compliance with existing accessibility guidelines.

Meta Tag Analyzer
This online tool will analyze your title tag, keyword meta tag, and description tag for proper construction and length.