Slide5-8 WCAG: Robustable Success Criteria

Robustable: Content must be robust enough that it can be interpreted reliably by a wide variety of user agents, like browsers and assistive technologies.
This means that users must be able to access the content as technologies advance. Meeting this requirement helps maximize compatibility with current and future user agents, like screen readers. In particular, it enables assistive technologies to process the content reliably, and to present or to operate it in different ways.
This includes non-standard buttons, input fields, and other controls. To deliver a desirable user experience, there must be a separation between web page design and user content. The web page may not render as expected in all browsers, and will not perform as expected in differing screen readers.

The intent of this Success Criterion is to ensure that user agents, including assistive technologies, can accurately interpret and parse content.

If the content cannot be parsed into a data structure, then different user agents may present it differently or be completely unable to parse it.
Some user agents use "repair techniques" to render poorly coded content.

Note: This criterion has been removed from WCAG 2.2.
In WCAG 2.1 and 2.0 this Success Criterion should be considered as always satisfied for any content using HTML or XML.