Recreate Templ's HTTP streaming w/ Shadow DOM using just the Go standard library #1288

tmaxmax · 2025-10-22T15:55:46Z

tmaxmax
Oct 22, 2025

Does anyone know how to recreate https://templ.guide/server-side-rendering/ streaming#declarative-shadow-dom using just the Go standard library?

I've tried to first render the main template which includes the <template shadowrootmode="open"> tag with the slot, call Flush on the response writer and then render the template which should fill in that slot.

That second templates ends up being rendered outside of the html tag. it looks like if the slot content isn't rendered as a sibling of the template the template slot isn't filled.

Does anyone here have any experience trying to stream content using shadow DOM and HTTP?

Thank you very much.

EDIT: Minimal reproducible example of my attempt:

package main

import (
	"io"
	"net/http"
	"time"
)

func main() {
	handler := func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Content-Type", "text/html")
		w.WriteHeader(http.StatusOK)
		io.WriteString(w, base)
		http.NewResponseController(w).Flush()
		time.Sleep(time.Second * 2)
		io.WriteString(w, fragment)
	}

	http.ListenAndServeTLS(
		":8080",
		"data/certs/localhost_fullchain.pem", "data/certs/localhost_privkey.pem", // for HTTP2
		http.HandlerFunc(handler),
	)
}

const base = `
<!DOCTYPE html>
<html>
	<head>
		<title>Hello world</title>
	</head>
	<body>
		<h1>Hello</h1>
		<div>
			<template shadowrootmode="open">
				<slot name="a">Loading A...</slot>
			</template>
		</div>
	</body>
</html>
`

const fragment = `
	<h2 slot="a">Success!</h2>
`

Could probably remove the flushing and sleeping as the issue comes only from how the HTML is handled.

a-h · 2025-10-25T09:04:07Z

a-h
Oct 25, 2025
Maintainer

I haven't looked at this in detail, but I notice that you close the HTML doc, then send the h2 element to fill the slot, whereas in the templ example, the slot contents are sent before the end html tag.

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Recreate Templ's HTTP streaming w/ Shadow DOM using just the Go standard library #1288

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Recreate Templ's HTTP streaming w/ Shadow DOM using just the Go standard library #1288

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a-h Oct 25, 2025 Maintainer

tmaxmax
Oct 22, 2025

a-h
Oct 25, 2025
Maintainer