Nibbler - Free tool for testing how good your website is, and what you can do to improve it. Check accessibility, SEO, social media, compliance and more.
Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
tota11y - An accessibility visualization toolkit from Khan Academy
FreeStylerWs - Styles and skins for all popular sites in the web
Accessible Metrics - Digital Governance
Stylem - A user style manager for Pale Moon, Basilisk, Interlink, SeaMonkey, and other Mozilla-based software. Install styles from userstyles.org to change how web pages and the application look.