I like this general proposal but I think we can go further! intercept and register don't need to be two different things; as the only difference between them is that register is for protocols that don't exist yet and intercept is for ones which do, which is something we can determine automatically.

I think we should just have one method, protocol.handle (thanks @dsanders11 for the name suggestion), that can either intercept an existing protocol or register a new one. Multiple handlers registered for the same protocol can chain. For example,

protocol.handle("app", async (request, next) => {
  const response = await getResponseFor(request)
  return response ?? next()
})

Here next would delegate to whatever existing handler there was, or else to some default failure behavior if there was no existing handler.

protocol.handle("http", async (request, next) => {
  if (request.path === '/')
    // I'm unsure if this should be something like "new StringResponse", or whether
    // a plain object is better
    return {content: '<body>hello world', mime: 'text/html'}
  else if (request.path === '/some/image.png')
    return {file: '/path/to/my/image.png'}
  else if (request.path === '/some/video.mp4')
    return {content: generateVideoStream(), mime: 'video/mp4'}
  else if (request.path === '/sneakily-cross-origin')
    return {url: 'https://foo.bar/baz'}
  else return next()
})

@nornagon has this design landed?

@samuelmaddock, protocol.handle landed which addresses most (all?) of the use cases from this PR, so I'll close this.