What happened

link to download on page gives 404.

where it says:

How to reproduce

go to the site https://facebook.github.io/immutable-js/

scroll to the paragraph:

To use Immutable.js from a browser, download dist/immutable.min.js or use a CDN such as CDNJS or jsDelivr.

click the link "dist/immutable.min.js"

it should give you a file, it gives 404