Guideline 4.1 – Compatible Maximize compatibility with current and future user agents, including assistive technologies. 4.1.1 Parsing Level A 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. Note 1: This Success Criterion should be considered as always satisfied for any content using HTML or XML. Note 2: Since this criterion was written, the HTML Living Standard has adopted specific requirements governing how user agents must handle incomplete tags, incorrect element nesting, duplicate attributes, and non-unique IDs. [HTML] Although the HTML Standard treats some of these cases as non-conforming for authors, it is considered to “allow these features” for the purposes of this Success Criterion because the specification requires that user agents support handling these cases consistently.