Reading random tweets from Daniel Glazman, I found this one, about the Bug 93725 - 'bolder' and 'lighter' keywords of 'font-weight' don't compound correctly. The bug itself is fascinating. How should be handled the interaction of fonts (inner) weight with regards to CSS properties. David Baron created test cases and code.
I checked the rendering in different browsers for one of the tests out of curiosity.
Opera 11 alpha