HTML Validation Services

The HTML Validation Service available from the World Wide Web Consortium (W3C) is on the web at:

HTML validation involves doing syntax checking and validation of the HTML within a single web page, it doesn't involve checking the hyperlinks to and from that page. There are several different major versions of the HTML standard (2.0, 3.2, and 4.0 are the most common versions used), and different browsers support different versions. By using an HTML validator, you can see when you are using incorrect HTML code that may work fine in one browser and not in another.

This seems to be the most up-to-date, and thorough, HTML validator that I've found on the web so far. A neat addition to this validator is the detailed explanation it can give for every problem it finds in a web page.