Wikipedia:Accessibility dos and don'ts

This page in a nutshell: All users, regardless of disability, should be able to read, navigate, and contribute to Wikipedia easily.

Therefore, when editing articles:

Do use high contrast and color-blind friendly color schemes.
Do provide alt text and a caption for most images.
Do provide a text description of any charts or diagrams.
Do nest headers sequentially.
Do create correctly structured tables.
Do encase non-English words or phrases in {{lang}}.
Do ask any accessibility-related questions at WikiProject Accessibility's talk page.

Don't impede the ability to distinguish links from regular text.
Don't use scrolling lists or collapsible sections to conceal content. However, "Collapsible sections or cells may be used in tables that consolidate information covered in the main text, and in navboxes", as explained in the related section of the Manual of Style.
Don't use color as the only means of conveying information.
Don't use tables for layout—use <div> blocks instead.
Don't leave blank lines between list items.
Don't use Unicode characters as icons or images.

See also