Many of them have either a couple of elements that depend on JS to show/hire correctly, or a big chunk of the page rendered as a single-page application.
In either case, the user interface isn’t “ready” after page load until JS has a chance to download, parse, and execute, which means core web vitals like largest contentful paint and cumulative layout shift are at risk. For your users, it means a flash of jank when they visit the page.
Some of the fixes for this are simple (ex: use CSS to hide elements that should be invisible by default), and others might get a little crazier (ex. prerender an embedded SPA into part of a page).
Alex MacArthur is a software engineer working for Dave Ramsey in
Soli Deo gloria.
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