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. (Level A)

Note: Start and end tags that are missing a critical character in their formation,
such as a closing angle bracket or a mismatched attribute value quotation mark
are not complete.

Sufficient Techniques for 4.1.1 – Parsing

• 1.G134: Validating Web pages
• 2.G192: Fully conforming to specifications
• 3.H88: Using HTML according to spec
• 4.Ensuring that Web pages can be parsed by using one of the following
  techniques:

• H74: Ensuring that opening and closing tags are used according to
  specification
  (HTML)
• H75: Ensuring that Web pages are well-formed
  (HTML)

Failures for SC 4.1.1 – Parsing

• F70: Failure of Success Criterion 4.1.1 due to incorrect use of
  start and end tags or attribute markup
• F77: Failure of Success Criterion 4.1.1 due to duplicate values
  of type ID
• F17: Failure of Success Criterion 1.3.1 and 4.1.1 due to insufficient
  information in DOM to determine one-to-one relationships (e.g., between labels
  with same id) in HTML
• F62: Failure of Success Criterion 1.3.1 and 4.1.1 due to insufficient
  information in DOM to determine specific relationships in XML

http://www.w3.org/TR/UNDERSTANDING-WCAG20/ensure-compat-parses.html