According to CSS Generated Content Module Level 3, §1.1, "Generated content should be (...) available to assistive technologies. The content property applies to speech and generated content must be rendered for speech output."

This is already the state of things for the most common usage of content within ::before and ::after pseudo elements. But for other use cases this makes less sense and might lead to unnecessary screenreader noise. For example, a ::marker's content will replace the native list marker which is purely presentational. Screenreaders currently don't read ::marker content which seems the right thing to do.