Out of order streaming with <template for>

[noamr]

Specification

whatwg/html#11818 (will end up in HTML standard)

Description

Allow updating exiting DOM using a template element

Documentation

https://github.com/WICG/declarative-partial-updates/blob/main/patching-explainer.md

Browser support

No response

[tunetheweb]

Suggest we use template-for as the feature id? Can we reserve that?

[tunetheweb]

Oh digging in I see that @foolip used this feature as an example in #3726 with the new "proposed" directory and README, temporarily named it html-pis.yml but did show of an example when this could be renamed to template-for.

@foolip (or anyone else), not sure if that was just an example, but any thoughts on which of those two names we want to use for this particular example? @noamr needs it for the I2S and I need it for the MDN docs.

[ddbeck]

Thanks for filing this. I'm taking some notes here to help someone author the YAML file for this feature. I'd welcome a review of this information from @noamr and @tunetheweb, but it's not required.

Name and ID

The name for this feature is probably something like Out-of-order HTML streaming.

Since an entry point is an attribute for the <template> element (feature ID template), template-for seems like a natural ID for this, though it would probably never be a candidate for merging back into template.

That said, because the processing instructions are also a kind of entrypoint for this, we might instead consider shadowing the name out-of-order-html-streaming or doing something like html-patching-processing-instructions to avoid suggesting that it's merely an extension of <template>'s behavior alone.

I'm still mulling this over and would welcome impressions on this.

Description

This is pretty novel, so it's a tough one to write. Here's my rough draft:

The <template for="…"> HTML element replaces the content at a named range or insertion point, marked by the processing instructions <?start name="…"> and <?end> or <?marker name="…">. You can use this to stream partial updates into the document out of order.

Compat data

I'm expecting these BCD keys:

- html.elements.template.for
- api.ProcessingInstruction
- api.ProcessingInstruction.ProcessingInstruction
- api.ProcessingInstruction.target
- api.ProcessingInstruction.hasAttributes
- api.ProcessingInstruction.getAttributeNames
- api.ProcessingInstruction.getAttribute
- api.ProcessingInstruction.setAttribute
- api.ProcessingInstruction.removeAttribute
- api.ProcessingInstruction.toggleAttribute
- api.ProcessingInstruction.hasAttribute

Since processing instructions are new, BCD doesn't have a convention for recording them. I'm going to talk to my collaborators on BCD, but I'm going to suggest something like:

- html.processing_instructions.end
- html.processing_instructions.marker
- html.processing_instructions.start

Browser support

I found https://chromestatus.com/feature/5111042975465472 and https://issues.chromium.org/issues/431374376, so it looks like this may appear in a release as soon as Chrome 150. This is an early feature, though only just.

I found two standards positions requests:

• Out of order streaming mozilla/standards-positions#1369
• Out of order streaming WebKit/standards-positions#628

I found no other implementation bugs.