Slide5-8 WCAG4: 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.
Ensure that Assistive Technologies (AT) can gather appropriate information about, activate (or set) and keep up to date on the status of user interface controls in the content.
When standard controls from accessible technologies are used, this process is straightforward. If custom controls have a different role and/or function than usual, then additional measures need to be taken to ensure that the controls provide important and appropriate information to assistive technologies and allow themselves to be controlled by assistive technologies.
Make users aware of important changes in content that are not given focus, and to do so in a way that doesn't unnecessarily interrupt their work. The scope of this Success Criterion is specific to changes in content that involve status messages.
The intended beneficiaries are blind and low vision users of assistive technologies with screen reader capabilities. An additional benefit is that assistive technologies for users with cognitive disabilities may achieve an alternative means of indicating (or even delaying or supressing) status messages, as preferred by the user.