1. Use alt tags/attributes.
2. Don't depend on colour to show anything of importance (eg. red images for warnings).
3. Use clear and simple language.
4. Use language markup for foreign words.
5. No flickering or blinking.
6. For data tables, identify row and column headers (<th>).
7. Check that your page looks ok with its stylesheets missing.
9. Don't use server-side image maps, and give client-side image maps a text equivalent.
10. If you use audio, video or animation to present content, also describe the content in plain text. Keep the descriptions up to date.
11. For more complicated data tables, get into HTML header and cell markup associations.
12. Title each frame, if you insist on using frames.
13. Provide an alternative text-only page if your original page is so hopelessly complicated that you don't stand a chance of making it accessible. Keep the alternative up to date.
This page is valid XHTML to WAI Double-A standard
UK accessibility intro + menu
UK accessibility site reviews
Comment on DRC Report
Simple WAI level A checklist
Level A -
TinHat Level A+
Tips for web editors
Non-HTML files (PDFs) and accessibility
Gov guidelines for UK gov sites (intro)