Ensuring content is robust

Based on the Robust principle:

Content must be robust enough that it can be interpreted by a wide variety of user agents, including assistive technologies.

Your content should be robust enough that it can be used by current and future technologies. Your content needs to be compatible with a variety of browsers and assistive technologies. This is absolutely important in order to make sure that assistive technologies can present content reliably and support different methods of operating. When content is robust, it doesn't matter what type of assistive technology is being used, it could be a screen reader, a refreshable braille display, or voice dictation, it should all work.

This sounds a lot more complex than it really is, but it is very important. Use proper semantics with good labels and you should be ok.

To be clear, accessibility is not a checklist or a tool, and doing everything exacly as documented here will not always guarantee an accessible, or compliant site. It's important to note that how you deliver accessibility is unique and completely dependant on your particular site content, and users.