[simiones]

You're getting a very clear error: your sample is not valid HTML, because, as the error informs you, HTML elements can't self close. Why would it reference some other document type for which this may or may not be valid syntax? Should it also tell you that these are not valid JSON objects or LISP forms?

Now, separately from that, it might be nice if it told you that it doesn't support DOCTYPE == XHTML and will treat the document as HTML anyway, but that should only appear as an error/warning on the DOCTYPE line, it shouldn't confusingly mention XHTML wherever you might have meant some XHTML-ism that is invalid HTML.

Also, XHTML is not "an older version of HTML", it is a completely different standard that is (or was) sometimes used instead of HTML.

[tstusr20190823]

"html_elements_cant_self_close" is not a very clear error. Yes, now it is clear what does it mean, and it would be great if other users should not start a new HN-discussion to receive clear explanations when it can be explained directly on "html_elements_cant_self_close" tooltip. We can polish text, decide to mention XHTML or not, but the error message should be more informative.