"Open Sans Light" renders broken on Windows

Today I had to learn that one of my favourite web fonts - the popular "Open Sans" (https://www.google.com/fonts/specimen/Open+Sans#charset) at font-weight: 300; - doesn't seem to be readable on Windows based systems and therefore shouldn't be used on websites!

At first I couldn't believe it as I tested the blog on Mac OS X, Ubuntu, iPhone, iPad, Nexus 7, Samsung S3 and Windows XP. But at Hacker News those were the first responses to one of my latest blog posts: https://news.ycombinator.com/item?id=6190580.

Luckily Robert Nix (@mischanix) created a pull-request for my blog at Github and added screenshots to prove the problem: https://github.com/aheusingfeld/aheusingfeld.github.com/pull/10

Thanks very much @mischanix for the pull-request. That's why I love open-sourced blogging!

It turns out that I used Firefox to test my page on Ubuntu and Windows XP. The iOS and Android devices had no problem in different browsers but on desktop systems other than Mac OS X the font rendered unreadable - except you were using Firefox!

PS: I really wonder whether the "MuseoSans-300" font used by #Coderwall suffers the same problems. Comments anyone?