W3C, or the World Wide Web Consortium, is a committee that determines the standards that are generally adopted for the Internet and for online content.
These standards include best practices for developers, accessibility standards, and privacy and security guidelines. The W3C develops resources, builds community on the web, and have working groups focused on establishing new standards as the web continues to evolve overtime.
The adoption of W3C standards are realized with support from major browsers such as Safari, Chrome, Firefox, and others, along with the designers, developers, and content creators who deliver standards-based products for these browsers and related platforms.
For details on web accessibility standards, you would reference the W3C’s WCAG, or Web Content Accessibility Guidelines.