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name: streaming-html description: "Use when reviewing server-side rendering setup, Next.js App Router pages, or Node.js HTTP handlers to verify that HTML is streamed incrementally rather than buffered until completion." metadata: category: performance priority: medium difficulty: advanced estimatedTime: "45" source: frontendchecklist.io url: https://frontendchecklist.io/rules/performance/streaming-html

Stream HTML to the browser before the full response is ready

Traditional SSR waits for all data fetches to complete before sending a single byte of HTML, which means the browser cannot parse, discover subresources, or render anything until the slowest data query finishes. Streaming decouples the delivery of above-fold content from slow database queries and third-party API calls, dramatically improving perceived performance and LCP.

Quick Reference

• Streaming sends HTML in chunks as it is generated instead of waiting for the full response
• React renderToPipeableStream and Next.js App Router stream by default when you use Suspense
• Place Suspense boundaries around slow data dependencies to flush above-fold HTML immediately
• Monitor TTFB in Lighthouse — values above 600ms are a sign that streaming is not being used

Check

Examine this server-side rendering implementation to determine whether HTML is streamed in chunks or buffered until fully generated before sending to the client.

Fix

Refactor the SSR implementation to use renderToPipeableStream with Suspense boundaries around slow data fetches, or use ReadableStream in a Node.js handler, so that above-fold HTML is flushed to the client immediately.

Explain

Explain how streaming HTML improves TTFB and FCP by decoupling the delivery of fast content from slow server-side data dependencies.

Code Review

Review server entry points and page components for renderToString calls, missing Suspense boundaries around data-fetching components, and use of await on slow queries before returning a response.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/rules/performance/streaming-html