This is not due to lax parsing though. It is due to the convention of how to process undefined tags and attribute: User agents must treat elements and attribut

This is not due to lax parsing though. It is due to the convention of how to process undefined tags and attribute: User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them. https://html.spec.whatwg.org/multipage/infrastructure.html#extensibility-2 https://html.spec.whatwg.org/multipage/infrastructure.html#e... This is unrelated to parsing - for example a XHTML parser will be strict and draconian in its parsing, but still process undefined tags and attributes as above.