it is not that the content can be parsed unambiguosly or not. That is a property of the language, not of the processor of the language. If the language is ambiguous then a parser could take arbitrary decisions in interpreting the sentences. Like Italian. HTML, not even 2.0, was never ambiguous. And if user agents were strict since the beginning, web authors would have never been allowed to write incorrect html.
what 411 should say is that authors should write syntactically correct code, which is what the techniques say.
To make this requirement easier to understand, we have reworded SC 4.1.1 to clarify that it must be possible to parse content without the need for user agent repair. The revised SC reads as follows:
4.1.1 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. (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 quote are not complete.