> How would you implement ability to arbitrarily block any network connection on any website without giving an extension 100% access?

Browsers should provide a filtering option before they makes a request.

IMO a lot of no-brainer options are missing from personal computers. Like the ability to start a program with restricted access to files, network or OS calls (on Windows and on Linux). Browsers should provide the ability to inspect, and filter network access, run custom javascript on websites, etc.

We do sort of have that with the capabilities stuff (although I admit hardly anyone knows how to use it).

But the tricky part is that "reading files" is done all the time in ways you might not think of as "reading files". For example loading dynamic libraries involves reading files. Making network connections involves reading files (resolv.conf, hosts). Formatting text for a specific locale involves reading files. Working out the timezone involves reading files.

Even just echoing "hello" to the terminal involves reading files:

  $ strace echo hello 2>&1 | grep ^open
  openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
  openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libselinux.so.1", O_RDONLY|O_CLOEXEC) = 3
  openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libgcc_s.so.1", O_RDONLY|O_CLOEXEC) = 3
  openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libm.so.6", O_RDONLY|O_CLOEXEC) = 3
  openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
  openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libpcre2-8.so.0", O_RDONLY|O_CLOEXEC) = 3
  openat(AT_FDCWD, "/proc/filesystems", O_RDONLY|O_CLOEXEC) = 3
  openat(AT_FDCWD, "/proc/self/maps", O_RDONLY|O_CLOEXEC) = 3
  openat(AT_FDCWD, "/usr/lib/cargo/bin/coreutils/echo/en-US.ftl", O_RDONLY|O_CLOEXEC) = -1 ENOTDIR (Not a directory)

OP says "restricted access to files". Read access to your home directory is not required for loading dynamic libraries or printing the time.

Capabilities are craaaazy coarse on Linux. Really only a small piece of the sandboxing puzzle. Flatpak, Bubblewrap, and Firejail each provide an overall fuller view of what sandboxing can be.

> the ability to start a program with restricted access to files, network or OS calls (on Windows and on Linux)

Bubblewrap allows you to do that on Linux.

I like that uBO can manipulate the dom, so that I can remove annoying divs or animations that I don't want. Its so much more than blocking network requests which I can do with DNS anyway.