It's a webkit thing. Try -webkit-font-smoothing: antialiased and you'll get the version with less weight, while 'subpixel-antialiased' is the heavier one. This weight difference is more noticeable in white text against dark backgrounds. Photoshop offers several options on how to render text that show different "weight" in a similar way... they're called "Crisp", "Strong", "Smooth", etc. In particular there's a "Mac L…
In other cases, this might be that. But in this particular case, it's not. If you look closely, it's not about the heaviness at the edges -- they're literally different outlines. Safari in this case is actually creating a "faux bold". It has nothing to do with the heaviness of antialiasing, like Photoshop's options.