font-display

auto

The font display strategy is defined by the user agent.

block

Gives the font face a short block period, generally around 3 seconds, and an infinite swap period.

swap

Gives the font face an extremely small block period and an infinite swap period.

fallback

Gives the font face an extremely small block period and a short swap period.

optional

Gives the font face an extremely small block period and no swap period.

The font-display descriptor for @font-feature-values determines the font display timeline; it does so by setting a default font-display value for @font-face for the same font-family name. When font-display is omitted in @font-face, the user agent first looks for the font-display value that has been set via @font-feature-values for the relevant font-family. If no value is found, the user agent uses the auto value for font-display, in which case, the user agent determines the font display strategy.

The font display timeline is based on a timer that starts when the user agent attempts to use a specific downloaded font face. The timeline is divided into three periods, as listed below. These periods dictate the rendering behavior of any element that uses the font face.

• Font block period: If the font face is not loaded, elements attempting to use the font are rendered with an invisible fallback font face. The browser blocks visible text rendering, saving a space for the to-be-displayed text based on metrics of the fallback font. During the block period, text is not visible. At the end of the block period, if the font has not loaded, the text is rendered in the fallback font.
• Font swap period: The swap period occurs after the block period (if there is one) and if the font face has not yet been loaded successfully. Elements attempting to use the not-yet-loaded font are rendered using the next available fallback font face. The formerly invisible fallback font face is painted onto the screen. If the font loads successfully during the swap period, the text that was rendered in the fallback font is updated — or swapped — with the newly loaded font. This step triggers a repaint.
• Font failure period: If the font face has not loaded by the time the swap period expires or by the time the block period expires (if there is no swap period, as is the case with optional), the user agent treats the font as a failed load. As a result, the content becomes visible in the fallback font.

The font-display descriptor allows you to set a default display policy for all @font-face rules, including those not under author control, such as third-party @font-face rules. When the font-display value is set via @font-feature-values, it becomes the default font-display value, and is applied to the entire font-family. However, any font-display value defined within individual @font-face blocks will override this default, but only for those blocks where the descriptor is defined.

@font-feature-values "Leitura" {
  font-display: swap;
  @swash {
    fancy: 1;
  }
}

The font-display descriptor in this example sets the default font-display value for the "Leitura" font to swap for all the @font-face blocks. Now there might be several @font-face blocks importing multiple font files for a single font-family. If one of those @font-face blocks includes a font-display descriptor, the specified value will be used for that specific font file only. All other blocks not including a font-display descriptor will default to swap instead of the user-agent's standard default auto.

There is no browser implementing this feature.

• font-display descriptor for @font-face at-rule
• CSS fonts module