W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
Dark Reader - Reduce eye strain in your browser with this extension that provides a dark theme for browsing.
Rocket Validator - Site-wide Accessibility and HTML validation made easy. With a single click, Rocket Validator will check thousands of web pages on your site for Accessibility and HTML conformance, producing a detailed, actionable report on seconds.
Stylus - User Styles Manager - Stylus is a userstyles editor and manager based on the source code of Stylish version 1.5.2.