Why are chrome, safari and firefox rendering Open Sans 400 as Extrabold Today half the web looks ugly. That's because half the web uses Open Sans 400, and on my macbook pro in all browsers it has suddenly started rendering as "Extrabold". Even on the Google Webfonts site the 400 weight version is rendering in the browser as extrabold. Baffled. 

I have already tried "Restore Standard Fonts" in Font Book.

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Not sure if you got an answer to this question, but I had exactly the same issue myself. Didn't have Open Sans open in Suitcase and it wasn't in my System Folder BUT I did find Open Sans Extrabold installed in my Creative Cloud assets using Typekit. I removed it, restarted machine and problem solved. Hope that helps if you hadn't gotten to the bottom of it!