Allow HTTP headers to be defined for a preload request

[IgorMinar]

The preload hints don't currently work if a SPA client makes an HTTP request via fetch or XHR apis to the server and in this request it specifies e.g. an Accept header in order to negotiate the format of the response.

A typical symptom of this issue is a warning about the preload request mismatch due to incompatible headers:

The root cause analysis in angular/angular#34899 (comment) revealed that the issue is indeed setting the Accept header.

Using the Accept header for content negotiation is a relatively common thing in the REST api design: https://restfulapi.net/content-negotiation/ as it allows a server to support multiple response formats using the same URL space.

It would be great if it was possible to specify the Accept header (and potentially other headers as well?) when configuring the preload hint to enable prefetching requests from servers that do use the Accept header for content negotiation.

[yoavweiss]

/cc @annevk
Is this something that came up in other contexts?

[annevk]

Ideally this follows from the destination (i.e., the as attribute), no? That's also roughly what the specification says today.

[yoavweiss]

@annevk - The destination determines the default Accept headers, but doesn't enable you to add custom ones. In this case, the OP wants to add custom accept to indicate a preference for JSON over XML, resulting in the request not being preloadable.

[annevk]

I see. I recall some prior discussion about trying to figure out how to put RequestInit into markup, but I cannot find it.

[cmmartin]

Maybe the type attribute could dictate the Accept header? So if you passed <link type="application/json", you'd get a "accept": "application/json", header instead of the current "accept": "*/*",.

It's not a complete solution, but the accept header seems to be the source of most issues. I can't prefetch anything, because I'll get XML if I don't provide a custom accept header

[yoavweiss]

Discussion continues at whatwg/html#7887