I use this line, or one like it, in a lot of quick demos. Not that it’s not a production-worthy line of code—I just tend to be a bit more explicit on bigger projects.

html {
  font: 110%/1.4 system-ui;
}

Someone wrote in confused by it, and I could see how a line like that is a bit bewildering at first.

The first thing to know is that it is called shorthand. The font property in CSS gives you the opportunity to set a bunch of font-* properties all at once. In this case, we’re setting:

html {
  font-family: system-ui;
  font-size: 110%;
  line-height: 1.4;
}

There are a few more little specific things to know. For example, the order matters.

/* invalid */
html {
  font: system-ui 100%/1.4;
}

You also can’t set the line-height without also setting the font-size. If you’re going to set line-height, you have to set both. Be extra careful there because something like 20px is both a valid line-height and font-size, and if you only set one, it’ll be the font-size. If you go for a unitless number, which is a great idea for line-height, and try to set it alone, it’ll just fail.

/* invalid */
html {
  font: 1.5 system-ui;
}

Of course, we’ve got all this detailed in the Almanac entry for font. But I’ll also give a shout to Mateusz Hadryś who has a detailed article titled “Full Text Styling With a Single Line of CSS” with some detailed figures like this that are helpful in understanding everything:

Showing the font property declaration. Style, variant and weight are in yellow with a label above that says order doesn't matter and a label beneath saying these have to be first. Next is size with a forward slash then line-height in red. Above them is a label that says these have to be next to each other. Next is family in light blue with a label that says it has to be last. There is an additional white label that connects size and family that says they are required.

Lastly, if system-ui was part of the confusion there, that’s one of those System Things.

Credit

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