[mwcampbell]

But can JavaScript check whether the protocol handler is installed? Or can it only attempt to use the protocol handler, then give the user if-then-else instructions to manually handle the case where it's not installed? Remember, a remote support product has to assume that the user receiving support doesn't have the knowledge or energy to go through a complex setup process, which is presumably a digression from whatever problem they were having in the first place.

[alex_suzuki]

It cannot. Enumerating protocol handlers is actually an excellent fingerprinting technique. That’s why platforms like iOS for instance forbid it, or you have to explicitly specify which ones you’ll query (see: https://developer.apple.com/documentation/uikit/uiapplicatio...).