Service Worker support

[jimmywarting]

I would like to be able to run less.js inside of a service worker...

By the looks of the source it seems that you are using god old XMLHttpRequest to fetch resources...

• Could this be replaced with fetch instead? if so then i suppose we could also drop this hole sync support?

It also seems as if you use LocalStorage to cache files...

• local/session storage don't work inside of worker, a better replacement would be either cache storage or indexeddb...