Stream HTML into element or ShadowRoot by noamr · Pull Request #12758 · whatwg/html

noamr · 2026-08-04T19:09:46Z

Add streaming HTML insertion methods to Element, ShadowRoot, and ChildNode

Introduce safe and unsafe streaming HTML parsing and insertion methods:

Element & ShadowRoot: streamHTML(), streamHTMLUnsafe(), streamAppendHTML(), streamAppendHTMLUnsafe(), streamPrependHTML(), and streamPrependHTMLUnsafe()
ChildNode: streamBeforeHTML(), streamBeforeHTMLUnsafe(), streamAfterHTML(), streamAfterHTMLUnsafe(), streamReplaceWithHTML(), and streamReplaceWithHTMLUnsafe()
This allows developers to parse and stream HTML directly into specific positions of the DOM tree using WritableStream.

The HTML fragment parser is divided into two:

Preparing the fragment parser (create an inert document with a proper element stack)
Using that parser to parse into a DocumentFragment

The preparation algorithm is used when streaming, by creating a new fragment parser, and then the stream is routed to feed that parser.

When parsing, the "insertion target redirection map" is configured to reroute insertions intended for the root to go directly into the stream's target element, instead of going into the DocumentFragment.

A special sanitizer mitigation is in place to avoid these removed elements from incurring side effects, by creating them in an inert document if they are about to be removed.

"Scripting mode" has been refactored so that the streaming fragment parser can execute scripts. Instead of relying on the fragment/inert/normal scripting mode, the parser receives a "target document" which is assigned to the script as an "intended document" and checked at preparation time. It is set to the dummy inert document for inert fragment parsing, and to the context document when runScripts is true.

Closes #2142

At least two implementers are interested (and none opposed):
- Chrome
- Gecko (HTML streaming & revamped DOM parsing mozilla/standards-positions#1370)
Tests are written and can be reviewed and commented upon at:
- Un-tentative streamHTML tests web-platform-tests/wpt#61748
Implementation bugs are filed:
- Chromium: Implemented
- WebKit: https://bugs.webkit.org/show_bug.cgi?id=315741
- Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2043150
Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
MDN issue is filed: Will be done as part of Blink shipping
The top of this comment includes a [clear commit message

(https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use.

(See WHATWG Working Mode: Changes for more details.)

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      <!DOCTYPE html>
      <html>
      <head>
          <meta name="viewport" content="width=device-width, initial-scale=1">
          <meta name="robots" content="noindex">
          <style>body,html{height:100%;margin:0}body{display:flex;align-items:center;justify-content:center;flex-direction:column;-webkit-font-smoothing:antialiased;text-rendering:optimizeLegibility}p{text-align:center;font-family:-apple-system,BlinkMacSystemFont,Segoe UI,Roboto,Oxygen,Ubuntu,Cantarell,Fira Sans,Droid Sans,Helvetica Neue,sans-serif;color:#000;font-size:14px;margin-top:-50px}p.code{font-size:24px;font-weight:500;border-bottom:1px solid #e0e1e2;padding:0 20px 15px}p.text{margin:0}a,a:visited{color:#aaa}</style>
      </head>
      <body>
      <p class="code">
        Error code: 503      </p>
      <p class="text">
        Well, This is unexpected. An Error has occurred, and we are working to fix the problem! We will be up and running shortly. Try refreshing the page or try again in a few minutes.
      </p>
        <div style="display:none;">
          <h1>
    upstream_reset_before_response_started{connection_termination} (503 UC)      </h1>
          <p data-translate="connection_timed_out">App Platform failed to forward this request to the application.</p>
      </div>
      </body>
      </html>

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See whatwg/html#12758 Note that tests that assert how this works with <template for> are still tentative as that PR is still not landed.

zcorpan

LGTM % spaces

noamr mentioned this pull request Aug 5, 2026

Un-tentative streamHTML tests web-platform-tests/wpt#61748

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noamr changed the title ~~Add streaming HTML insertion methods~~ Streaming HTML insertion methods Aug 5, 2026

noamr changed the title ~~Streaming HTML insertion methods~~ Stream HTML into element or ShadowRoot Aug 5, 2026

noamr force-pushed the noamr/stream-html branch from df24ea2 to e416732 Compare August 5, 2026 09:32

noamr closed this Aug 5, 2026

noamr reopened this Aug 5, 2026

zcorpan mentioned this pull request Aug 5, 2026

Integrate fragment parser options with trusted types #12583

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noamr force-pushed the noamr/stream-html branch from e416732 to 5106e9d Compare August 6, 2026 16:23

noamr force-pushed the noamr/stream-html branch from deb55bb to e97c374 Compare August 6, 2026 19:46

noamr force-pushed the noamr/stream-html branch from d5a18a7 to b19b385 Compare August 7, 2026 20:17

zcorpan reviewed Aug 11, 2026

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noamr force-pushed the noamr/stream-html branch from d87ab8d to 4880d38 Compare August 11, 2026 15:49

noamr force-pushed the noamr/stream-html branch from 2de75ec to ce9f49b Compare August 11, 2026 20:18

noamr added a commit to web-platform-tests/wpt that referenced this pull request Aug 12, 2026

Un-tentative streamHTML tests

5478eb6

See whatwg/html#12758 Note that tests that assert how this works with <template for> are still tentative as that PR is still not landed.

zcorpan approved these changes Aug 12, 2026

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noamr force-pushed the noamr/stream-html branch from 421aa3f to 043dcdf Compare August 12, 2026 13:02

noamr force-pushed the noamr/stream-html branch from 043dcdf to 286c499 Compare August 12, 2026 13:25

noamr added 6 commits August 12, 2026 17:08

Stream HTML

f9ef56a

Nits + fix IDL

6c178e8

nit

183c3f9

Remove CEReactions from methods that create a stream

fd13000

Move root script check to 'initialize'

cc74af9

Line breaks

ce45065

Remove extra spaces

146266b

noamr force-pushed the noamr/stream-html branch from 286c499 to 146266b Compare August 12, 2026 16:08

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Stream HTML into element or ShadowRoot - #12758

Stream HTML into element or ShadowRoot#12758
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noamr commented Aug 4, 2026 •

edited by pr-preview Bot

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noamr commented Aug 4, 2026 •

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