Summary: if you're specifying a bold font style (700) but not supplying a bold version of the font (only 400 and 600), Safari creates a "faux bold" on its own, while other browsers fall back to the next-closest weight (600).
Neither behavior seems more obviously correct, since you're asking for something that doesn't exist.
Moral of the story: whenever you're using webfonts that come in a variety of weights, make sure you're loading font files in the weights you're intending to use! Otherwise behavior may vary.
(And in contrast to multiple comments here, this has nothing to do with macOS using different/darker types of antialiasing than other operating systems. That is not what this article is about.)
Neither behavior seems more obviously correct, since you're asking for something that doesn't exist.
Moral of the story: whenever you're using webfonts that come in a variety of weights, make sure you're loading font files in the weights you're intending to use! Otherwise behavior may vary.
(And in contrast to multiple comments here, this has nothing to do with macOS using different/darker types of antialiasing than other operating systems. That is not what this article is about.)