Created attachment 468769 [details]
Deno Server which streams HTML. Run with `deno run --allow-net deno-server-streaming-html.js`.

Safari doesn’t seem to be streaming HTML at all, except for `<iframe>` elements. Right now, it appears to buffer all the HTML before rendering anything.

To reproduce you will need a web server that supports streaming. The attached example uses Deno (1.38.2). When run locally, it’ll serve HTTP/1.1 and adds the correct `Transport-Encoding: chunked` header. I have a hosted variation of the example which is being served with Deno Deploy: https://shadowroot-streams.deno.dev. It also uses the Declarative Shadow DOM. That example uses HTTP/2 and correctly doesn’t use the `Transport-Encoding` header since it’s incompatible with HTTP/2. In short, Deno’s streaming the content correctly and Deno Deploy supports HTML streaming.

Note: the attached example admittedly doesn’t have a tonne of HTML in the first chunk, but that doesn’t seem to matter at all for streaming into an `<iframe>` element. Other browsers are also able to stream this fine.