Web Accessibility in Mind (WebAIM), a group the forefront of universal design practices, offers easy-to-understand walkthroughs of ways to make your web content more accessible.
WebAIM also created WAVE, a website accessibility evaluator, which anyone can use to see specifically where accessibility issues appear on a website.
W3C (World Wide Web Consortium) offers a great collection of design tips and tricks through their Web Accessibility Initiative, and also has an HTML markup validator tool, which identifies coding errors on a particular website.
Supporting information comes from WebAIM (Web Accessibility in Mind).
"How to Evaluate a Website" - The Visual Communication Guy Blog
"Measuring Accessibility in the User Experience (UX) and the Searcher Experience" - Marketing Land
"Web Content, Universally Designed" - Accessibility at Colorado State University