Implement <input type="url">

[ferjm]

Spec: https://html.spec.whatwg.org/multipage/forms.html#url-state-(type=url)

May block 13942

[jdm]

Won't block; the input types gracefully degrade to regular text inputs.

[gpoesia]

I'd like to work on this. It's mostly about adding a new InputType and implementing proper validation in components/dom/htmlinputelement.rs (and probably handling parsing as well somewhere else), right? What are the other changes I'm missing?

One question about this part of the spec:

User agents may allow the user to set the value to a string that is not a valid absolute URL, but may also or instead automatically escape characters entered by the user so that the value is always a valid absolute URL (even if that isn't the actual value seen and edited by the user in the interface).

On allowing input that is not a valid absolute URL, it seems there's some room for different handlings. Should the automatic escaping be implemented? Is it talking about e.g. percent-encoding spaces while the user types?

[jdm]

It's not clear to me what we can actually implement without first implementing #11444.