Alternative text on images allows assistive technology like screen readers to understand the purpose of an image on a page or allow them to skip it if purely decorative.
Events like toasts and status messages are visually communicated to GitHub users. Making sure those announcements are read via assistive technology allows users with low or no vision to get that status information.
Headings play a critical role in communicating the structure of a page. Find out why they're critical and how to create an accessible hierarchy in your pages.