Reviewing the below items will help get you started with CSS HTML Validator.

hmtoggle_plus1Validations default to "Errors only" mode to show only errors
hmtoggle_plus1Getting the most from the integrated editor
hmtoggle_plus1Preview documents or browse the web while checking the source with the integrated web browser
hmtoggle_plus1Automatically tidy up, reformat, or pretty print HTML documents and fix simple problems
hmtoggle_plus1How do I save or print the validation messages?
hmtoggle_plus1What do the colored squares in the tabs mean?
hmtoggle_plus1Things to know about the built-in link checker
hmtoggle_plus1See also