Understanding WCAG2.0

There should be no error in markup describing web content.

In addition to analyzing this accessibility checker, you can also use Markup Validation Service.

Principle Robust Content must be robust enough that it can be interpreted reliably by a wide variety of user agents, including assistive technologies.
Guideline Compatible Maximize compatibility with current and future user agents, including assistive technologies.
Criterion Parsing
(4.1.1 A)
In the content implemented using the markup language, the element has a complete start tag and end tag, the elements are nested according to the specification, the element does not have duplicate attributes, any ID Is also unique. However, excluding those approved by the specification.

URL of Techniques for WCAG 2.0