In order to be compatible with the w3 spec for
custom url schemes it would be good if the chosen URI was prefixed with
web+ e.g.
web+bitcoin://
This will be an epic failure for w3c if it goes through like this. Be sure to write them and tell them how ridiculous these custom URI rules are. More important to comply with the long-standing IETF RFCs, which we already do.
Sorry but I see no evidence that the web+ prefix is required, or even suggested by w3c.
This prefix seems to be requested only by Chrome, according to the page linked above.
I made some tests:
I was able to register a website handling "bitcoin:" in Firefox, using javascript.
The same javascript failed in Chrome, not because of the prefix, but because the method is not defined.
Then you can have
only a website register itself: normal/sane software is not allowed to register for these "web+" URIs.
You can register the bitcoin: protocol with a desktop app and a website. The popup window lets you choose when you click on a bitcoin: link. I tested it.