Professional HTML/CSS syntax checker/validator

CSE HTML Validator is a
fast, powerful, inexpensive, highly user configurable, and easy to use
professional HTML development tool that will help you to write syntactically
correct HTML. You'll be surprised how many problems may exist in your HTML!
HTML Validator will help you locate and fix them so that your documents are
easily viewed by a variety of web browsers. In addition, HTML Validator will
give you valuable and customizable advice based on your HTML document. CSE
is great for both HTML beginners and professionals who want to make sure
that their website visitors do not run into problems caused by poorly
written HTML. Version 5.0 includes link checking, CSS syntax checking, and a powerful, easy to
use, integrated HTML/text editor.