Thank you for the comments.
I envisioned something really simple. I would be perfectly satisfied if values were limited to those starting with "http://" and "https://", that makes it a bit simpler to determine the address may be _usable_, which means it isn't necessarily valid.
As far as it being valid, I'd personally put the burden entirely on the programmer, none at all on the editor, IOW, aside from establishing the text starts with "http://" or "https://" in order to change the cursor to the one the system normally uses to indicate a url, nothing else. IOW, the editor would not concern itself with validity at all, i.e, no fancy url parsing.
There is one thing that, on second thought, might be appropriate if this feature is implemented and that is, it should be an "opt-in" feature because it is conceivable for it to be used maliciously. Because of this, I believe it should be inactive by default and activated only by the programmer when he/she is willing to accept full responsibility for whatever actions a link may cause.