Subtitles are written words on a screen that share dialogue of an audiovisual program (TV show, movie etc.) in real time, used by those unable to understand the spoken language of a video.
Sign language is a natural language where people communicate using hands and bodily gestures instead of their mouth, primarily used by people who are deaf, hard of hearing, have hearing difficulty, or are unable to vocalize speech.
A screen reader is a type of assistive technology that outputs on-screen text using text-to-speech, primarily used by those who are blind.
Semantic Markup refers to marking up documents in ways that provide information about the content itself rather than information about the visual styling of the content. It is critical to ensure that assistive technology users can understand your document.