HTML Guides for font-display

font-display controls how a font face is displayed based on whether and when it has been downloaded and is ready to use. Its accepted values — auto, block, swap, fallback, and optional — determine the behavior during the font-loading timeline (block period, swap period, and failure period). Because it governs the loading behavior of a font face definition, it only makes sense within a @font-face rule, not as a property applied to an HTML element.

When you place font-display inside a regular selector like body or .heading, the CSS validator correctly flags it as an unknown property. Browsers will silently ignore it, meaning your intended font-loading strategy won't take effect. Users may experience a flash of invisible text (FOIT) or other unwanted behavior because the browser falls back to its default font-display strategy.

Why this matters

• Font-loading performance: Without a valid font-display descriptor in your @font-face rule, you lose control over how the browser handles text rendering while custom fonts load. Using font-display: swap, for example, ensures text remains visible with a fallback font until the custom font is ready — a key web performance best practice.
• Standards compliance: The CSS Fonts Module Level 4 specification defines font-display exclusively as a @font-face descriptor. Using it elsewhere produces invalid CSS.
• Silent failure: Browsers won't throw visible errors — they simply ignore the invalid property, which can make it difficult to diagnose why your font-loading behavior isn't working as expected.

How to fix it

1. Remove font-display from any regular CSS rule (selectors like body, h1, .class, etc.).
2. Add font-display inside the @font-face at-rule where you define your custom font.
3. If you're loading fonts via a third-party service (like Google Fonts), you can often append a &display=swap parameter to the font URL instead of writing your own @font-face block.

Examples

❌ Incorrect: font-display used as a CSS property

body{

font-family:"Open Sans", sans-serif;

font-display: swap;

}

This triggers the validator error because font-display is not a valid CSS property for element selectors.

✅ Correct: font-display used inside @font-face

@font-face{

font-family:"Open Sans";

src:url("/fonts/open-sans.woff2")format("woff2"),

url("/fonts/open-sans.woff")format("woff");

font-display: swap;

}

body{

font-family:"Open Sans", sans-serif;

}

Here, font-display: swap is correctly placed inside the @font-face rule, telling the browser to immediately render text with a fallback font and then swap in "Open Sans" once it finishes loading.

✅ Correct: Using display=swap with Google Fonts

If you use Google Fonts, you don't need to write your own @font-face block. Instead, add the display parameter to the URL:

<linkrel="stylesheet"href="https://fonts.googleapis.com/css2?family=Open+Sans&display=swap">

Google Fonts will generate the @font-face rules with font-display: swap included automatically.

❌ Incorrect: font-display in a class selector

.heading{

font-family:"Roboto", sans-serif;

font-display: optional;

}

✅ Correct: Separate the descriptor from the styling rule

@font-face{

font-family:"Roboto";

src:url("/fonts/roboto.woff2")format("woff2");

font-display: optional;

}

.heading{

font-family:"Roboto", sans-serif;

}

The font-display: optional descriptor now correctly lives in the @font-face block, where it tells the browser to use the custom font only if it's already cached — otherwise, stick with the fallback. The .heading rule simply references the font family by name.