The things that are invalid should all have defined behaviour. For example, a <label> is not allowed to contain two form controls, but is defined as applying to

The things that are invalid should all have defined behaviour. For example, a <label> is not allowed to contain two form controls, but is defined as applying to the first such control. As far as parse errors is concerned, https://html.spec.whatwg.org/multipage/parsing.html#parse-errors https://html.spec.whatwg.org/multipage/parsing.html#parse-er... says: > This specification defines the parsing rules for HTML documents, whether they are syntactically correct or not. Certain points in the parsing algorithm are said to be parse errors. The error handling for parse errors is well-defined (that's the processing rules described throughout this specification), but user agents, while parsing an HTML document, may abort the parser at the first parse error that they encounter for which they do not wish to apply the rules described in this specification.