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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 it’s usually undesirable. That’s tampered by doing something like font-size: calc(16px + 1vw). But while we’re getting fancy with calculations anyway, the most common implementation ended up being an equation to calculate plain English:

I want the type to go between being 16px on a 320px screen to 22px on a 1000px screen.

Which ended up like this:

html {
  font-size: 16px;
}
@media screen and (min-width: 320px) {
  html {
    font-size: calc(16px + 6 * ((100vw - 320px) / 680));
  }
}
@media screen and (min-width: 1000px) {
  html {
    font-size: 22px;
  }
} 

That’s essentially setting a minimum and maximum font size so the type won’t shrink or grow to anything too extreme. “CSS locks” was a term coined by Tim Brown.

Minimum and maximum you say?! Well it so happens that functions for these have made their way into the CSS spec in the form of min() and max().

So we can simplify our fancy setup above with a one-liner and maintain the locks:

html {
  font-size: min(max(16px, 4vw), 22px);
}

We actually might want to stop there because even though both Safari (11.1+) and Chrome (79+) support this at the current moment, that’s as wide as support will get today. Speaking of which, you’d probably want to slip a font-size declaration before this to set an acceptable fallback value with no fancy functions.

But as long as we’re pushing the limits, there is another function to simplify things even more: clamp()! Clamp takes three values, a min, max, and a flexible unit (or calculation or whatever) in the middle that it will use in case the value is between the min and max. So, our one-liner gets even smaller:

body {
  font-size: clamp(16px, 4vw, 22px);
} 

That’ll be Chrome 79+ (which doesn’t hasn’t even dropped to stable but will very soon).

Uncle Dave is very happy that FitText is now a few bytes instead of all-of-jQuery plus 40 more lines. Here is us chucking CSS custom properties at it:

See the Pen
FitText in CSS with clamp() by Dave Rupert (@davatron5000)
on CodePen.

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