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Chrome for Developers
Blog
Declarative partial updates
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Barry Pollard
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Noam Rosenthal
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Published: May 19, 2026
The web has long since moved on from the static, document-driven medium that it started as. Modern, rich web apps are used by everyone for many reasons, from communicating, purchasing, consuming rich content, to managing our complex lives.
HTML, despite all its advances, is still delivered in-order in a top-to-bottom fashion with little regard for when content is ready or when the user consumes it. CSS lets you change the ordering of content, but often with significant accessibility side effects. JavaScript lets you manipulate the DOM through various APIs to break free of this somewhat, but those often require verbose syntax or construction of DOM trees to plug into HTML.
Performance is incredibly important for the web, given the client-server nature of the medium but suboptimal choices are often made to circumvent this in-order nature of HTML, which slows down performance. This includes waiting until the whole page is ready or using a heavy framework to deliver components in an asynchronous manner. The popularity of JavaScript frameworks shows that web developers prefer a component-based model rather than the rigid document mental model of the web's origins.
The Chrome team has been considering this problem and has been developing new additions to the web platform under the name of Declarative Partial Updates.
Two new sets of APIs make it easier to deliver HTML in a less linear fashion, whether out-of-order in the HTML document itself or through easier ways to dynamically insert HTML into existing documents using new JavaScript APIs. These are ready for developer testing from Chrome 148 using the chrome://flags/#enable-experimental-web-platform-features flag. Polyfills are also available to let you use these new APIs right away, even in browsers that don't yet support them.
These additions to the web platform are being standardized with positive feedback from other browser vendors and standardization avenues. The relevant standards are in the process of being updated to include these new APIs.
Out-of-order streaming
The first set of changes are new out-of-order streaming APIs using the HTML element and processing instruction placeholders. For example:
...
Here is some HTML content!
Processing instructions have existed in XML for a long time, but have been treated as comments in HTML and ignored. This new API changes that and brings processing instructions to HTML. When the browser sees the processing instructions, it doesn't do anything straight away—much like before—but they can be referenced later.
The element looks up the corresponding processing instructions with a name attribute and replaces the content. In this case, after being parsed, the DOM ends up as:
Here is some HTML content!
As well as the attribute for replacements, there are also and range markers which allow for temporary placeholder content to be shown before the template is processed:
Loading…
...
Here is some HTML content!
In this case, Loading… shows until the is seen and then is replaced with the new content.
It is also possible to include processing instructions in templates to allow multiple updates:
Loading…
...
Result One
...
Result Two
...
This results in the following HTML after being parsed:
Result One<br>Result Two
With the final processing instruction at the end in case any more are added to the document later.
Demo
In this video, a basic photo album application is implemented with streaming HTML:
Photo album demo implemented with out-of-order streaming (source)
Both the status and photos are streamed into the HTML after the initial layout.
Use cases
There are many use cases for this out-of-order patching HTML when coupled with streaming HTML:
Island architecture. A common pattern popularized by frameworks like Astro the island architecture where components are hydrated independently on top of static HTML. The API lets static content be handled in a similar fashion directly in HTML. JavaScript frameworks can also use this for more interactive islands or to handle components.
Delivery content when it is ready. Thanks to this island architecture, content can be streamed when it is ready rather than held back for content that requires extra processing, for example, a database lookup. While many platforms allow streaming HTML, the in-order nature of HTML means that content is often held back, or by resorting to complex JavaScript...