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Remove leftover critical CSS on hydration mismatch #13995

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Merged
merged 2 commits into from
Jul 15, 2025
Merged

Remove leftover critical CSS on hydration mismatch #13995

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d4ee0fa
Remove leftover critical CSS on hydration mismatch
markdalgleish Jul 15, 2025
f1c4c0b
Update .changeset/lucky-hotels-complain.md
brophdawg11 Jul 15, 2025
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5 changes: 5 additions & 0 deletions .changeset/lucky-hotels-complain.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
"react-router": patch
---

[REMOVE] (continuation of #13872) In Framework Mode, remove leftover critical CSS elements in development after initial render if there's a hydration mismatch
17 changes: 17 additions & 0 deletions packages/react-router/lib/dom-export/hydrated-router.tsx
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Expand Up @@ -24,6 +24,7 @@ import {
UNSAFE_hydrationRouteProperties as hydrationRouteProperties,
UNSAFE_createClientRoutesWithHMRRevalidationOptOut as createClientRoutesWithHMRRevalidationOptOut,
} from "react-router";
import { CRITICAL_CSS_DATA_ATTRIBUTE } from "../dom/ssr/components";
import { RouterProvider } from "./dom-router-provider";

type SSRInfo = {
Expand Down Expand Up @@ -242,6 +243,22 @@ export function HydratedRouter(props: HydratedRouterProps) {
setCriticalCss(undefined);
}
}, []);
React.useEffect(() => {
if (process.env.NODE_ENV === "development" && criticalCss === undefined) {
// When there's a hydration mismatch, React 19 ignores the server HTML and
// re-renders from the root, but it doesn't remove any head tags that
// aren't present in the virtual DOM. This means that the original
// critical CSS elements are still in the document even though we cleared
// them in the effect above. To fix this, this effect is designed to clean
// up any leftover elements. If `criticalCss` is undefined in this effect,
// this means that React is no longer managing the critical CSS elements,
// so if there are any left in the document, these are stale elements from
// the original SSR pass and we can safely remove them.
document
.querySelectorAll(`[${CRITICAL_CSS_DATA_ATTRIBUTE}]`)
.forEach((element) => element.remove());
}
}, [criticalCss]);

let [location, setLocation] = React.useState(router.state.location);

Expand Down
13 changes: 11 additions & 2 deletions packages/react-router/lib/dom/ssr/components.tsx
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Expand Up @@ -212,6 +212,8 @@ function getActiveMatches(
return matches;
}

export const CRITICAL_CSS_DATA_ATTRIBUTE = "data-react-router-critical-css";

/**
Renders all of the `<link>` tags created by route module {@link LinksFunction} export. You should render it inside the `<head>` of your document.

Expand Down Expand Up @@ -247,10 +249,17 @@ export function Links() {
return (
<>
{typeof criticalCss === "string" ? (
<style dangerouslySetInnerHTML={{ __html: criticalCss }} />
<style
{...{ [CRITICAL_CSS_DATA_ATTRIBUTE]: "" }}
dangerouslySetInnerHTML={{ __html: criticalCss }}
/>
) : null}
{typeof criticalCss === "object" ? (
<link rel="stylesheet" href={criticalCss.href} />
<link
{...{ [CRITICAL_CSS_DATA_ATTRIBUTE]: "" }}
rel="stylesheet"
href={criticalCss.href}
/>
) : null}
{keyedLinks.map(({ key, link }) =>
isPageLinkDescriptor(link) ? (
Expand Down