a11y.css is a Firefox extension designed to assist developers in identifying accessibility issues within their web applications. Originally a bookmarklet, this tool has evolved to include a comprehensive set of features aimed at enhancing the accessibility of web content. By integrating a specific stylesheet, it highlights potential risks and errors in HTML code, making it easier for developers to ensure compliance with accessibility standards.

In addition to its core functionality, a11y.css offers various tools such as outlining focus for keyboard navigation, displaying languages used on the page based on language attributes, and checking for the presence of alt and title attributes on images. This extension serves as a valuable resource for developers looking to create more accessible web experiences, providing essential insights into potential improvements.