What’s the deal with declaring font properties on @font-face?

I hope you read that title out loud in your best Seinfeld impression.

A recent question in our forums made me aware that there are more properties that can be added to @font-face than the usual font-family and src suspects. What are the point of those? Why would you want to declare other font declarations there?

I'm used to writing @font-face essentially like this:

@font-face {
  font-family: "My Custom Font";
  src: url("path/to-font-file.woff2");
}

But the spec does indeed include more descriptors:

@font-face {
  font-family: <family-name>;
  src: <url>;
  unicode-range: <urange>;
  font-variant: normal | [ <east-asian-variant-values> || <east-asian-width-values> || ruby ];
  font-feature-settings: normal | <feature-tag-value>;
  font-stretch: normal | ultra-condensed | extra-condensed | condensed | semi-condensed | semi-expanded | expanded | extra-expanded | ultra-expanded;
  font-weight: normal | bold | bolder | lighter | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900;
  font-style: normal | italic | oblique;
}

You'll see this plenty when looking at @font-face code generated by font services.

They are the gatekeepers!

That's an analogy you can use to think about @font-face declaration blocks. The font will be downloaded and used, if it passes the gatekeepers.

The font-family is the obvious one. You're declaring a new font-family to use, so any element that wants to use this font needs to have a matching one.

@font-face {

  /* Gatekeeper: font-family on another element must match this to be used. */
  font-family: 'Open Sans';

  src: url(my-font.woff2) format('woff2');

}

body {
  /* Match! */
  font-family: 'Open Sans';
}

h1, h2 {
  /* No match */
  font-family: 'Different Font';
}

They are (mostly) all gatekeepers

With exception to font-variant and font-feature-settings, all of the properties can be used as filters that instruct the browser to download and use the font files when the values match.

@font-face {

  /* Only download and use if element matches this font-family */
  font-family: 'My Font';

  /* Only download and use if element matches this font-style */
  font-style: italic;

  /* Only download and use if element matches this font-weight */
  font-weight: 700;

  src: url(my-bold-and-italic-font.woff2) format('woff2');

  /* Only download and use if these characters are used */
  unicode-range: U+0460-052F, U+20B4, U+2DE0-2DFF, U+A640-A69F;
}

Maybe a diagram explanation will be helpful

A diagram showing HTML, the CSS selectors that match that HTML, and the @font-face declration blocks that ulitmately apply.

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