[#669] Declarative partial update use case #958
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| name: out-of-order-html-streaming | ||||||||||||||
| description: Declaratively update a page with content streamed later into the same HTML document or via imperative JavaScript APIs. | ||||||||||||||
| web-feature-ids: | ||||||||||||||
| - template-for | ||||||||||||||
| - tmp-html-setters | ||||||||||||||
| - tmp-html-streaming-setters | ||||||||||||||
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| --- | ||||||||||||||
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| # Out-of-Order (OOO) HTML Streaming | ||||||||||||||
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| Out-of-Order (OOO) HTML Streaming (also known as Declarative Partial Updates) is an alternative to the traditional "top-to-bottom" linear processing of HTML. It allows developers to define placeholders earlier in a document and "patch" them with content that arrives later in the same network stream, or via new imperative JavaScript streaming APIs. | ||||||||||||||
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| ## Quick Overview | ||||||||||||||
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| The feature consists of two primary mechanisms: | ||||||||||||||
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| 1. **Declarative Patching (HTML-based)**: Uses new processing instructions (`<?marker>`, `<?start>`, `<?end>`) and `<template for>` to swap content in the DOM as it streams in from the server. | ||||||||||||||
| 2. **Imperative setting and streaming (JS-based)**: A suite of new methods (e.g., `streamHTML()`, `setHTMLUnsafe()`) that allow setting of HTML or piping readable streams directly into DOM elements with native performance and optional script execution. | ||||||||||||||
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| ## Best Practices | ||||||||||||||
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| * **Prioritize Above-the-Fold Content**: Stream critical UI (header, hero, primary navigation, primary content) first. Use placeholders for "expensive" or lower priority components like comments, ads, or related content. | ||||||||||||||
| * **Implement Native Loading States**: Use the `<?start>` and `<?end>` markers to wrap fallback content. The browser automatically replaces everything between these markers when the corresponding `<template for>` arrives. | ||||||||||||||
| * **Minimize Cumulative Layout Shift (CLS)**: Always provide stable dimensions for placeholders. If the final content size is unknown, use skeleton screens or CSS `aspect-ratio` to reserve space and prevent jarring shifts. | ||||||||||||||
| * **Accessibility & Focus Management**: Be mindful that out-of-order updates can confuse screen reader users. Use `aria-live` regions for critical updates and ensure that focus isn't lost if a placeholder containing the active element is replaced. | ||||||||||||||
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| ## Implementation Details | ||||||||||||||
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| ### 1. Declarative HTML Markers | ||||||||||||||
| The browser looks for specific processing instructions to identify where content should be patched. | ||||||||||||||
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| ```html | ||||||||||||||
| <!-- A simple insertion point --> | ||||||||||||||
| <div id="sidebar"> | ||||||||||||||
| <?marker name="dynamic-comments"> | ||||||||||||||
| </div> | ||||||||||||||
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| <!-- A replacement range with loading state --> | ||||||||||||||
| <main> | ||||||||||||||
| <?start name="main-content"> | ||||||||||||||
| <div class="skeleton">Loading content...</div> | ||||||||||||||
| <?end> | ||||||||||||||
| </main> | ||||||||||||||
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| ... (other content) ... | ||||||||||||||
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| <!-- Later in the same stream, provide the content --> | ||||||||||||||
| <template for="dynamic-comments"> | ||||||||||||||
| <div class="comments">...</div> | ||||||||||||||
| </template> | ||||||||||||||
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| <template for="main-content"> | ||||||||||||||
| <article> | ||||||||||||||
| <h1>The Real Content</h1> | ||||||||||||||
| <p>This replaces the skeleton div automatically.</p> | ||||||||||||||
| </article> | ||||||||||||||
| </template> | ||||||||||||||
| ``` | ||||||||||||||
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| Once a `<template>` replaces an insertion point, that insertion point no longer exists, and can not be reused. However, a `<template>` may contain insertion points that can be used later in the stream. | ||||||||||||||
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| ```html | ||||||||||||||
| <?start name="replaced-content"> | ||||||||||||||
| <div class="skeleton"> | ||||||||||||||
| <?end> | ||||||||||||||
| <!-- Later in the same stream, provide initial content --> | ||||||||||||||
| <template for="replaced-content"> | ||||||||||||||
| <?start name="replaced-content"> | ||||||||||||||
| <div>Quickly computed result</div> | ||||||||||||||
| <?end> | ||||||||||||||
| </template> | ||||||||||||||
| <!-- Later in the same stream, provide full content --> | ||||||||||||||
| <template for="replaced-content"> | ||||||||||||||
| <?start name="replaced-content"> | ||||||||||||||
| <div>Full result</div> | ||||||||||||||
| <?end> | ||||||||||||||
| </template> | ||||||||||||||
| ``` | ||||||||||||||
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| ### 2. Imperative JS Streaming | ||||||||||||||
| You can now pipe fetch responses directly into the DOM without manual chunk handling. | ||||||||||||||
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| ```javascript | ||||||||||||||
| const main = document.querySelector('main'); | ||||||||||||||
| const response = await fetch('/api/partial-update'); | ||||||||||||||
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| // Pipe the stream directly into the element | ||||||||||||||
| await response.body | ||||||||||||||
| .pipeThrough(new TextDecoderStream()) | ||||||||||||||
| .pipeTo(main.streamHTMLUnsafe({ runScripts: true })); | ||||||||||||||
| ``` | ||||||||||||||
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| ## Use Case Reference Matrix | ||||||||||||||
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| * **Island Architecture**: Hydrate independent interactive components on top of static HTML without a full framework runtime. | ||||||||||||||
| * **Deferred "Expensive" Blocks**: Deliver the shell of a page immediately and stream database-heavy components as they become available. | ||||||||||||||
| * **Single Page Application (SPA) Updates**: Stream large UI transitions in SPAs using `streamHTML()` instead of waiting for a full JSON payload and manual DOM construction. | ||||||||||||||
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| ## Anti-Patterns & Warnings (DO NOT DO THIS) | ||||||||||||||
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| * **Do not use OOO streaming for everything.** Over-fragmenting a page can increase the complexity of the stream and potentially delay content if the browser spends too much time patching small nodes. | ||||||||||||||
| * **Do not ignore Trusted Types.** When using `*Unsafe` methods in environments with Trusted Types enforced, ensure you are passing a sanitized policy object if required. | ||||||||||||||
| * **Do not assume script execution order.** Scripts inside a `<template for>` will execute as soon as they are patched into the document. Do not rely on them executing in a specific sequence relative to other deferred templates. | ||||||||||||||
| * **Do not use without a polyfill strategy.** Ensure you have a fallback or use polyfills (`html-setters-polyfill` and `template-for-polyfill`) to support your required browsers until this feature becomes Baseline. | ||||||||||||||
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| ## Fallback strategies | ||||||||||||||
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| {{ BASELINE_STATUS("template-for") }} | ||||||||||||||
| {{ BASELINE_STATUS("html-setters") }} | ||||||||||||||
| {{ BASELINE_STATUS("html-streaming-setters") }} | ||||||||||||||
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There was a problem hiding this comment. You could prefix these with
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There was a problem hiding this comment. That doesn't work in the (It even fails if you comment out (
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There was a problem hiding this comment. Oh interesting, that sounds like a bug. Let me know if you want to merge this as-is and this is the only blocker, then I can look into a fix. Otherwise the easiest thing would be to wait until the feature ID is published. Alternatively you could do something like this, and in the next web-features update PR, we'll get a CI check telling us to remove the tmp prefix and we can resolve this TODO while we're in there.
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There was a problem hiding this comment. No rush to merge this. Especially as half of it isn’t available until 154 anyway. Though the fact you have to use polyfills for other browsers anyway means you could use it now. Anyway let’s five it a little longer.
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There was a problem hiding this comment. This is released, so I've removed the tmp features. We'll need to wait until web-features gets updated to 3.39.0 on this repo. |
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| You can use polyfills like `template-for-polyfill` and `html-setters-polyfill` to emulate the API surface for non-supporting browsers. | ||||||||||||||
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