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CSS Font Blocking and Calls

November 2025

font-display

  • auto: whatever the user agent picks
  • block: short block - infinite swap
  • swap: extremely short block - infinite swap
  • fallback: extremely short block - short swap
  • optional: extremely short block - no swap

Timings

According to this sentry.io post the timings are 100ms for the extremely short period and 3s for the short period. I don't see where they got those numbers though.

To look at: 
font-size-adjust,
size-adjust, ascent-override, descent-override, and line-gap-override.

https://blog.sentry.io/web-fonts-and-the-dreaded-cumulative-layout-shift/

https://codepen.io/simonjhearne/pen/rNMGJyr

https://deploy-preview-15--upbeat-shirley-608546.netlify.app/perfect-ish-font-fallback/?font=Miltonian
end of line

References

Lists the extremely short time period as 100ms.