Web Site Design

There is some great advice on accessibility at Dos and don'ts on designing for accessibility | Accessibility. In addition Inclusive - Microsoft Design is well worth looking at.

Cross Browser Testing

It is increasingly important to test websites on different platforms and with different browsers but once you start looking at specific devices then the permutations are huge. So use resources like Cross browser testing - Learn web development | MDN for help you and learn more. In addition Mozilla have been highlighting Free Firefox Testing on Real Devices recently (Feb 2017).

Icons

It is always nice if you website icon appears on your browser tab and when pinned in the operating system. So check out Favicon Generator for all platforms: iOS, Android, PC/Mac... for help building icons and HTML to support this.

Handy Links

Hardening Content Security Policy (CSP) - good beginner tips
HTTP Observatory (Beta) - Mozilla Header checking
Analyse your HTTP response headers - HTTP Header Scanner
WebAIM: Alternative Text
Make the Web Work For Everyone ★ Mozilla Hacks – the Web developer blog