Pretext

Simon Willison’s Weblog Subscribe Sponsored by: WorkOS — Ready to sell to Enterprise clients? Build and ship securely with WorkOS. Pretext (via) Exciting new browser library from Cheng Lou, previously a React core developer and the original creator of the react-motion animation library. Pretext solves the problem of calculating the height of a paragraph of line-wrapped text without touching the DOM. The usual way of doing this is to render the text and measure its dimensions, but this is extremely expensive. Pretext uses an array of clever tricks to make this much, much faster, which enables all sorts of new text rendering effects in browser applications. Here's one demo that shows the kind of things this makes possible: The key to how this works is the way it separates calculations into a call to a prepare() function followed by multiple calls to layout(). The prepare() function splits the input text into segments (effectively words, but it can take things like soft hyphens and non-latin character sequences and emoji into account as well) and measures those using an off-screen canvas, then caches the results. This is comparatively expensive but only runs once. The layout() function can then emulate the word-wrapping logic in browsers to figure out how many wrapped lines the text will occupy at a specified width and measure the overall height. I had Claude build me this interactive artifact to help me visually understand what's going on, based on a simplified version of Pretext itself. The way this is tested is particularly impressive. The earlier tests rendered a full copy of the Great Gatsby in multiple browsers to confirm that the estimated measurements were correct against a large volume of text. This was later joined by the corpora/ folder using the same technique against lengthy public domain documents in Thai, Chinese, Korean, Japanese, Arabic, and more. Posted 29th March 2026 at 8:08 pm Recent articles Vibe coding SwiftUI apps is a lot of fun - 27th March 2026 Experimenting with Starlette 1.0 with Claude skills - 22nd March 2026 Profiling Hacker News users based on their comments - 21st March 2026 This is a link post by Simon Willison, posted on 29th March 2026. browsers 102 css 234 javascript 748 testing 93 react 41 typescript 16 Monthly briefing Sponsor me for $10/month and get a curated email digest of the month's most important LLM developments. Pay me to send you less! Sponsor & subscribe Disclosures Colophon © 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026