Fluid typography is the idea that
font-size (and perhaps other attributes of type, like
line-height) change depending on the screen size (or perhaps container queries if we had them).
The core trickery comes from viewport units. You can literally set type in viewport units (e.g.
font-size: 4vw), but the fluctuations in size are so extreme that's usually undesirable. Let's look at a technique for tamping the extremes, but it basically a one-liner now that CSS is evolving!