Safari's "Reader Mode" - Open source solution?

Safari's reader mode is based upon Arc90's Readability(the original, not current, one), which is Apache-licensed. Here's a PHP port of it. Some more searching will probably turn up various other languages. (You didn't really specify where you were asking about implementing.)

I'm aware of the licencing issue about my post might raise here, but it's Apple and their policies are philosophically contestable. So, for the common good & research :

Safari JS reader source: https://github.com/amumu/safari-reader-js

How to get the latest version from Safari is explained in the README.

If you really want an open source Version, user the other posts, they're all good.

Cheers.

Here is a fairly popular JavaScript version based on arc90's readability project as well.

https://github.com/luin/readability