Parsing
In content implemented using markup languages, elements have complete start and end tags, elements are nested according to their specifications, elements do not contain duplicate attributes, and any IDs are unique, except where the specifications allow these features.
In plain language
WCAG officially lists this criterion as "Obsolete and removed" as of WCAG 2.2. It used to require clean, well-formed code: every tag properly opened and closed, elements nested correctly, no duplicate attributes, and no two elements on the same page sharing the same ID.
Why it matters
It doesn't need active attention anymore. This criterion existed because older assistive technology used to parse HTML directly, so broken markup could confuse it. Today, assistive technology relies on the browser's own parsing instead of doing its own, and browsers already handle messy or malformed HTML reliably and consistently. Any real accessibility problems that broken markup could still cause are now covered by other criteria, like Info and Relationships and Name, Role, Value.
In practice
Fails this criterion
No fail example here — this criterion has been removed from WCAG 2.2 and is no longer tested on its own.
Meets this criterion
There's nothing to check for this criterion specifically anymore, but the original requirement is still good practice: every tag has a matching closing tag, elements are nested correctly, and no two elements on the same page share the same id.
Test it yourself
You don't need to test this separately anymore. If your product already passes Info and Relationships (1.3.1) and Name, Role, Value (4.1.2), you're covered.