How I Learned to Stop Worrying and Ignore WCAG

1.1.1 Non-text Content (A) Continued

Exceptions to Non-Text Content

Test
If the alt text would invalidate the purpose of the test, then the text alternative should describe the purpose.
Sensory
If you have content that creates a specific sensory experience then alt text needs to provide a descriptive identification of the content.
CAPTCHA
alt text must say that its a CAPTCHA and alternative CAPTCHAs for different senses must be provided
Decoration, Formatting, Invisible
If the content is purely decorative, used for visual formatting, or not presented to users, it must be ignored by assistive technology. (i.e. It should not have a text alternative)

Examples