Those two screenshots are actually using different fonts (you can tell by the shape of the bottom part of the 'g', for example: in one screenshot it's the same

Those two screenshots are actually using different fonts (you can tell by the shape of the bottom part of the 'g', for example: in one screenshot it's the same width as the top part, but in the other it's wider). Chances are, this is a difference in what the different browsers do with the "garamond-premier-pro" font family name and which font they actually end up using when that's specified. Any chance you could confirm that?