Lets assume you designed something for desktops on a 1440px artboard and your text-size is supposed to be 16px. This mixin allows you to hit that target but mix in some screen-width to make sure your fonts scale nicely on larger or smaller screens.
font-size: screen-ratio-mix(
.2,
// 20% of the the font-size is relative to the screen width
1440,
// artboard width in PX
16,
// target PX size (base font-size in the design)
// 16px only for the demo, I usually set 10px as a base so 1rem would be around 10px
);// would also work in CSS only, but would be even harder to read
// results in:
font-size: calc(80% + 0.1388888889vw);
Make sure the view-width ratio is not too hight, since it makes it less accessible. (Having a bigger font-size set in your browser would not do anything, if the font-size was solely expressed in vw)