Support relative URLs

Attached patch. It mirrors the current state of the merge request.

I have a problem with your definition of "length".
Here we are using an "iframe"-element defined in https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe
So that values as plain number (e.g. 701) are (1) normal usage and (2) implicite an px-Value

So we would need for *ratio* another approach.
Whats about an explicite rule: width="ratio" or width="4:3" simply *width: auto* height will then be ignored and the respocive-scheme is used.

Fixed in 8.x-2.23

Automatically closed - issue fixed for 2 weeks with no activity.