Font weight ignored in Chrome
This seems to be a Chrome/Chromium bug, caused by having the font installed locally on your system. Other browsers don't seem to suffer from this issue.
So far, it seems to occur on Linux and Windows (confirmed).
For some reason, it will just load your local font and ignore any of your font-weight rules, even if they're !important. It won't even be consistent with itself: the font weight can change randomly between tabs and page reloads.
The simplest workaround is to remove the font, but that could be an issue if you need it for something else.
You might also try renaming the font to something else in order to force Chrome to use your web font and honour your CSS font rules.
Try changing the font family to 'Open Sans Light', sans-serif;. I had the same problem and this worked for me.