Using an HTML validator helps ensure that your document is free of syntax errors that can result in unexpected output when users who view your syntactically incorrect documents view them with different browsers. Testing your HTML documents on a browser does not test for HTML validity. Invalid HTML documents may be treated differently by different browsers and cause your page to look like something you never intended. It can even cause your page to render in some browsers but not render at all in others. Making sure that your documents are syntactically correct helps to make sure that your documents are always viewed the way that you want them to be-- and that they are written the way that they should be. A valid document also looks good to someone who is looking at your document to see how well you have designed it, both aesthetically and syntactically.

Finds misspelled or invalid tag names, tag attributes, tag attribute values, and character entities
Finds missing and mismatched quotation marks
Finds missing or extra closing tags
Detects incorrect placing and nesting of tags
Alerts you to incorrect or incompatible HTML based on a specific browser or HTML version (for example, notifies you of many Netscape specific or Internet Explorer specific tags, attributes, and attribute values)
Alerts you to the use of deprecated tags and attributes
Alerts you to bad style and can tell you what to do to improve your style
Finds broken links
Check multiple files at once with the Batch Wizard
When HTML Validator finds problems, you can fix them easily with the integrated HTML/Text editor.