Some details on how it works from a code comment:

Problem: DOM-based text measurement (getBoundingClientRect, offsetHeight) forces synchronous layout reflow. When components independently measure text, each measurement triggers a reflow of the entire document. This creates read/write interleaving that can cost 30ms+ per frame for 500 text blocks.

Solution: two-phase measurement centered around canvas measureText.

prepare(text, font) — segments text via Intl.Segmenter, measures each word via canvas, caches widths, and does one cached DOM calibration read per font when emoji correction is needed. Call once when text first appears.

layout(prepared, maxWidth, lineHeight) — walks cached word widths with pure arithmetic to count lines and compute height. Call on every resize. ~0.0002ms per text.

https://github.com/chenglou/pretext/blob/main/src/layout.ts