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.
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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.
---
For full implementation details, code examples, and framework-specific guidance,
see `references/rule.md`.
Rule page: https://frontendchecklist.io/rules/performance/streaming-html
