XHTML came after HTML 4.0, so it had its advent around the rise of JavaScript's popularity. However, HTML 2.0, which you can find specified here [0], and it sp

XHTML came after HTML 4.0, so it had its advent around the rise of JavaScript's popularity. However, HTML 2.0, which you can find specified here [0], and it specifies the "ignore" behaviour. Short of spinning up an old VM, I think I'll trust that my memory hasn't failed me. From RFC 1866: > markup in the form of a start-tag or end-tag, whose generic identifier is not declared is mapped to nothing during tokenization. Undeclared attributes are treated similarly... > For example: > <div class=chapter><h1>foo</h1><p>...</div> > => <H1>,"foo",</H1>,<P>,"..." [0] https://www.w3.org/MarkUp/html-spec/ https://www.w3.org/MarkUp/html-spec/