The current requirement fo unambiguous parsing is impossible to meet -
there are always multiple ways of parsing any data.
Instead I suggest the old wording of "conforms to formal published
This makes it feasible for User Agents to recognise the content and parse
it either according to the rules for such grammars, or in cases where it
is necessary (such as HTML) to use specifically adapted parsing techniques.
To make this requirement easier to understand, we have reworded SC 4.1.1 as follows:
Content implemented using markup languages has elements with complete start and end tags, except as allowed by their specifications, and are nested according to their specifications.
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 quote are not complete.
The working group looked at this topic carefully over an extended period of time and concluded that requiring strict adherence to all aspects of specifications does not necessarily result in an increase in accessibility. For example, it is possible to create invalid pages that present no accessibility barriers. It is also possible in certain situations to enhance accessibility through the use of markup that is not part of the specification.
The working group must work within its charter and only include things that directly affected accessibility. Some aspects of "use technologies according to specification" and validity do relate to accessibility. However, others do not. So requiring validity would take us beyond our charter. We do recommend it though and it is our #1 technique listed for conforming to SC 4.1.1.