ReferenceError: Worker is not defined

I’m trying out the library “greenlet”

https://runkit.com/crazy4groovy/5a7fa942eaeb7a00120bf78e

I get an error:
ReferenceError: Worker is not defined
at module.exports in greenlet/dist/greenlet.js — line 1

Note - it works well on jsfiddle: https://jsfiddle.net/developit/mf9fbma5/