[selectors-4] update HTML link?

[annevk]

It's now https://html.spec.whatwg.org/multipage/semantics-other.html#selectors. The old one redirects, but it's without HTTPS and relies on JavaScript to redirect fully.

[fantasai]

Fixed, thanks! (Also went through and updated the rest of the links.)

I couldn't find a target for the previously-existing dropzone attribute, though, so I deleted the sentence referring to it... I suspect the drag-and-drop section needs a thorough review (and possible deletion) from someone who understands the HTML feature's current state.