Programmatically specifying a document language lets screen readers know how to properly pronounce your content.
Be sure to also identify sections where the language changes, such as quotes or foreign phrases. If your app is localized, the programatic identification should update when the language updates.
Most images should include an alternative description, for those who cannot see it. An alternate (also known as alt) description should succinctly describe the content of the image.
The W3C’s WAI Web Accessibility Tutorials provides an alt decision tree to help you determine what images need a description.
Video content that includes audio should display synchronized captions. Caption content should include speech, but also relevant sound effects and music.
Transcripts are also important to provide, especially for longer-form audio such as podcasts.
Checkboxes, radio buttons, and single and multiline text inputs all need to have a label programmatically associated with them. The label provides a description of what the input is for. Labels should be concise and describe what kind of input the form represents (ex:“Phone number”).
Links should point to a valid URL, and buttons should trigger activity. Link and button content should be clearly and concisely written. Link content should provide enough context about its destination. Button content should describe what kind of action will be triggered when activated.