All in favor 👍
...now that browser, Node and Deno all also supports native (lazy loading) ES modules. (all doe, this package have nothing to do with server side code and is only meant to run on the main thread)

Do you have any suggestion how to keep it backwards compatible?
leave the streamsaver.js as is and just copy and make a new streamsaver.esm.js?

I'm going to make a major release making this a ESM-only module. and remove all cjs umd stuff and only use ESM

It will require to change streamsaver.js into using ESM export syntax, the other files mitm.html and sw.js don't need any change. so it's just one file.

Something like this needs to be added to package.json

  "type": "module",
  "exports": {
    "./*.js": "./*.js"
  },

also remove the "main" entry point from package.json. i want to be explicit about what path you can import.
so it should be:

- import streamsaver from 'streamsaver'
+ import streamsaver from 'streamsaver/StreamSaver.js'

...And the README examples needs to be updated.
could be nice to mention if someone are stuck with cjs then they could install the current release version.

it's important to test out that this settings still works:

// StreamSaver can detect and use the Ponyfill that is loaded from the cdn.
streamSaver.WritableStream = streamSaver.WritableStream
streamSaver.TransformStream = streamSaver.TransformStream
// if you decide to host mitm + sw yourself
streamSaver.mitm = 'https://example.com/custom_mitm.html'

i think it will be alright if we do export default ... instead of named exports cuz those are live bindings and acts more like a constant variable, so it can't be changed in ESM easily unless it's a default object that is exported