7. Design & Browser Compatibility
7.1 Modern Support
Websites must utilize HTML5 and work on modern browsers.
7.2 Structure & Layout
Websites must not use tables as the primary means of site structure of layout.
7.3 External Stylesheets
Websites must utilize external CSS stylesheets for styling.
Best Practices
While the use of inline styles can sometimes be necessary, excessive use of inline styles is not permitted.
7.4 Favicons
Websites must provide at least a 16px by 16px favicon.
7.5 Development Environments & Resources
Production websites must not serve assets such as images, CSS, and Javascript from development environments.
7.6 Flash
Websites must not use Flash.
Security Vulnerabilities & End-of-Life
Not only does Flash have numerous security vulnerabilities, Adobe, in collaboration with large web and computer companies, have stopped updating and distributing Flash Player as of December 31, 2020.
For more information regarding Flash's end-of-life, check out Adobe's Flash & The Future of Interactive Content and Flash Player EOL General Information articles.