Most faculty include some externally produced content in their course required course materials. This could be articles from the web, YouTube videos, publisher resources, and software. If you are requiring your students to use it, you need to be sure it is accessible.
This video explains the essentials for ensuring outside materials are accessible.
In addition to watching the video, please look through the Additional Resources. Then you will be ready for the following assessments.
Most electronic textbooks are accessible, and they also tend to be cheaper. Just remember to check before you buy.