Chrome and firefox are rendering incorrectly OPEN SANS web font
I tested in latest Windows 10 (64-bit) on latest Firefox 48 and Chrome 52, and do not see like the screenshots above. And the Dev Tools shows Open Sans being loaded.
Have you also tried including the various font styles? Since your missing them from your CSS stylesheet href on your <link>.
It should be something like this so Google knows what font styles to deliver
<link rel="stylesheet" href="http://fonts.googleapis.com/css?family=Open+Sans:400,600,300,800,700,400italic" />Google Web Font page has like 10 font styles for Open Sans
https://fonts.google.com/specimen/Open+Sans?query=open+sans
Look and see if the various font styles appear, if they do not it could be your graphics card or some kind of Windows 8 bug causing that kind of rendering.
UPDATE
Also check if ClearType is affecting the text in your browser. Try enabling and disabling it and see if you see a difference. In windows search for Adjust ClearType Text.
https://www.microsoft.com/typography/cleartype/tuner/tune.aspx