When the font size is set to 600 or higher, it appears bold, which is what was expected. It is possible that if a thinner version of the font were installed, the lower sizes would also work. Without that option, the browser defaults to normal or bold.
According to the Mozilla CSS Reference:
If the exact weight specified is not available, the following guidelines are used to determine the rendered weight:
- For weights greater than 500, choose the closest darker weight available (or lighter if none).
- For weights less than 400, select the closest lighter weight available (or darker if none).
- For a weight of 400, use 500 if available; otherwise follow the guidelines for weights less than 400.
- For a weight of 500, use 400 if available; otherwise follow the guidelines for weights less than 400.
This implies that for fonts with only normal and bold options, 100-500 represent normal weights while 600-900 signify bold weights.
Edit: It seems that support for lighter weights is also present.