What complicated things would a permissions system not be able to handle?

Here's an analogy: HTTPS Everywhere wants to, quote, "Read and change all your data on the websites you visit."

I mean, yes, that's what it's doing. But a permission system that allowed it to say "Rewrite URL references for a, img, style, video, etc. tags, modifying the protocol, except where you also need to modify other things like s|^http://(\w{2})\.wikipedia\.org/wiki|https://secure.wikimedia... would essentially be presenting me with the source code for HTTPS Everywhere to approve.

And I certainly don't want it prompting on each website, which would be the natural way to implement a permission dialog system. Remember in the late '90s when web browsers would ask you for every cookie, or prompt you when going from HTTP to HTTPS?

Meanwhile, if a remote-code-execution bug is ever found in HTTPS Everywhere, it will have access to literally everything I do on the web. So it's not clear the permission system is really helping anything.

I don’t think the prompt should continually appear. It would just appear the first time an app needs to do something that’s currently out-of-bounds for its sandbox. You then accept it or decline it. Much how the iOS system works.

But what is "something"? Is saying "yes, you can modify google.com" permission to modify news.ycombinator.com too? If so, then you're giving every extension permission to mess with every website, which defeats the point of a permission scheme. If no, HTTPS Everywhere is prompting on each new website, which is unbearable.

It doesn’t defeat it. I expect a browser extension—whose job it is to rewrite URLs—to be able to rewrite URLs. But I don’t expect HTTPS Everywhere to want access to my camera, or to my filesystem, or to the microphone. So I allow the first one, and get very worried if I’m ever even asked about any of the others.

Another browser extension might want to automatically save images I come across to a directory. That one would prompt for access to my Documents folder. It wouldn’t request URL rewrite privileges, or camera, etc.

EDIT: So to answer your question, what is “something”? In this case, that would be: “HTTPS Everywhere wants to be able to edit the web addresses you visit”. Or something like that.

Something like "Read and change all your data on the websites you visit."

It can't just be the web addresses you visit, since it needs to change embedded tags inside the page to request https urls.

Basic Internet Access. How often will you read the dialog for Chrome before you just go "Screw it, set to Always"?

Just once, the first time the application runs into a particular sandbox boundary. The application can decide for itself what granularity it wants to request, then I accept or decline.

For example, the first time I use Chrome to visit a website, it would trigger a dialog requesting permission to “Connect to websites” (i.e. initiate outbound TCP connections toward ports 80 and 443).

The first time I tried to use WebRTC, the dialog would appear for that.