I made a small polyfill on top of path-to-regex to play around with this and found a few differences.

First of all, something like

pathname: '/*.:imagetype(jpg|gif|png)'

doesn't work, the wildcard has to be (.*) instead. I do like the /* wildcard so maybe we can just document this as an improvement.

You expose named groups as result.pathname.groups - but JS regexp actually supports named groups now (path-to-regex doesn't use that feature) and that it exposed to result.groups.

Basically this

const imagePattern = new URLPattern({
  pathname: '/(.*).:imagetype(jpg|gif|png)'
});

let result = imagePattern.exec("/photo.jpg");

console.log(result)
console.log(result.pathname.groups['imagetype']);

could be done with regexps like:

const imageRegex = new RegExp("/(.*)\.(?<imagetype>jpg|gif|png)");
let res = imageRegex.exec("/photo.jpg");

console.log(res);
console.log(res.groups['imagetype']);