The W3C's Markup Validation Service is an invaluable tool for anyone who writes HTML. It pinpoints problems with HTML pages, and it gives clear, helpful error messages. I use it constantly to fix my own pages as well as pages that authors submit to Scheme Requests for Implementation. Most authors write HTML by hand, and there are a lot of errors to fix.
Uploading pages to the Validator, then manually jumping to each error
in Emacs was tedious, so I wrote validate-html, a simple command that automates the process. It uses Emacs's
standard next-error command to jump from one error to the next. It's now available for
installation through MELPA, and the source code is on Github.
In thanks for the Validator and everything else that the W3C does, I
made a donation through their W3C Friends program. I hope you'll find validate-html useful, and I hope you'll consider donating, too.