Make sure all elements have complete start and end tags, that they're nested properly, have no duplicate attributes and that IDs are unique, in order to make sure assistive technology can properly parse your content.
4.1.1 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. (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.
How to fix:
The easiest way to address this checkpoint is to run your markup though one or more validators, such as the W3C's, (http://validator.w3.org/ ) or a validator built into your authoring tool (for example, BBEdit's built in validator). These tools will help you find misnested tags, duplicate IDs, or other malformed markup.