> congratulations, the text now looks something like 50% bigger than you intend it to be
Some of these "stacks" do make poor choices with respect to matching the overall metrics of the fonts, but there are any number of reasons why a font might display bigger or smaller than you "intend it to be" in a web browser. Web design fundamentally has to account for this.
Yes, and that's exactly the problem with this approach. There's no accounting for the different metrics of the fonts.
Font size and font family should ideally always be set together. If you're setting a font-size, then you should force a font-family, and vice versa. Otherwise, you should set neither and let the user agent decide. This ensures that you would never get a broken combination that's uncomfortable or unusable.
It's similar to the idea that whenever you're setting a foreground color, you should always set the background color as well.
Some of these "stacks" do make poor choices with respect to matching the overall metrics of the fonts, but there are any number of reasons why a font might display bigger or smaller than you "intend it to be" in a web browser. Web design fundamentally has to account for this.