Out-of-order HTML streaming moves from frameworks into the browser
Serguey Asael Shinder2 min readOut-of-order HTML streaming — showing and allowing interaction with a page before it has fully loaded — is being moved out of JavaScript frameworks and into the browser itself. InfoQ reported the state of the proposal on 21 September 2026. Chrome 150 ships the declarative features; Safari and Firefox have signalled support.
The declarative half works through the standard <template> element and a new for attribute. A page declares an insertion point with a processing instruction such as <?marker name="profile">, or wraps temporary fallback content between <?start name="profile"> and <?end>.
When the server later streams a <template for="profile"> in the same document, the HTML parser matches the identifier, removes the start and end markers along with any fallback nodes between them, and inserts the template's contents at that position. Processing instructions inside templates allow a placeholder to be updated more than once. Matching JavaScript APIs accompany the declarative form.
What it means
The pattern is not new — this is what streaming SSR in the major frameworks has been doing for years, with each framework carrying its own runtime to do it. Moving the mechanism into the parser removes that runtime from the critical path, and with it the requirement that the client execute framework code before the first placeholder can be filled.
The part to watch is not capability but portability. A feature that ships in one engine while two others have signalled support is a feature with a support matrix, and the fallback story — what a browser without it renders when the markers arrive — decides whether anyone can use it outside a controlled client base. Signalled support is also not shipped support; the useful posture now is to read the proposal and to test against Chrome 150, not to rewrite a rendering layer around it.