I recently had an interesting conversation about the use of the HTML 4 transitional doctype, and was surprised to find out that it was no longer valid. I thought this website was the official / final say on the matter, but apparently it is both outdated and "just an informative reference", whatever that means: https://www.w3.org/QA/2002/04/valid-dtd-list.html
This is true despite the fact that the best validator I know of, https://validator.w3.org/nu/, agreed this was a valid doctype. But apparently that was a mistake as well, and they recently fixed it so it shows that doctype as an error.
So what does it mean for HTML to be versionless? Here is the best I can understand it. The doctype is not supposed to alter the way the browser behaves. Browsers should continue to support outdated elements, attributes, etc for backwards compatibility and to give the web time to catch up to the latest standards. Authors should stop using them. To the extent browser choose to support outdated stuff, they do so as a single version of HTML, the same way they support "HTML 5" stuff.
But if HTML is now versionless, why are we using the terms HTML 5.1 and 5.2?
This is true despite the fact that the best validator I know of, https://validator.w3.org/nu/, agreed this was a valid doctype. But apparently that was a mistake as well, and they recently fixed it so it shows that doctype as an error.
So what does it mean for HTML to be versionless? Here is the best I can understand it. The doctype is not supposed to alter the way the browser behaves. Browsers should continue to support outdated elements, attributes, etc for backwards compatibility and to give the web time to catch up to the latest standards. Authors should stop using them. To the extent browser choose to support outdated stuff, they do so as a single version of HTML, the same way they support "HTML 5" stuff.
But if HTML is now versionless, why are we using the terms HTML 5.1 and 5.2?
References:
https://blog.whatwg.org/html-is-the-new-html5
https://html.spec.whatwg.org/multipage/syntax.html#the-doctype
https://github.com/validator/validator/issues/408
https://github.com/htacg/tidy-html5/issues/466