Add `Sec-Fetch-*` Fetch Metadata headers.

[mikewest]

Please confirm that:

• You have read and understood the requirements for registration.
• You have checked the registry and found no current value that meets your needs.
• Your specification reference URL is stable; ideally, managed by a widely-recognised standards development organisation (e.g., published as an RFC). Otherwise, please give additional information below.

If so, please provide the details of your request below:

---

As outlined in https://www.w3.org/TR/fetch-metadata/#iana (and with @martinthomson's helpful reminder in whatwg/fetch#1818 (comment)), I'd like to register four headers that are currently widely deployed by user agents as request headers: Sec-Fetch-Dest, Sec-Fetch-Mode, Sec-Fetch-Site, Sec-Fetch-User.

Sec-Fetch-Dest

• Field name: Sec-Fetch-Dest
• Status: Permanent
• Structured Type: Item
• Specification document(s): https://www.w3.org/TR/fetch-metadata/#sec-fetch-dest-header
• Comments:

Sec-Fetch-Mode

• Field name: Sec-Fetch-Mode
• Status: Permanent
• Structured Type: Item
• Specification document(s): https://www.w3.org/TR/fetch-metadata/#sec-fetch-mode-header
• Comments:

Sec-Fetch-Site

• Field name: Sec-Fetch-Site
• Status: Permanent
• Structured Type: Item
• Specification document(s): https://www.w3.org/TR/fetch-metadata/#sec-fetch-site-header
• Comments:

Sec-Fetch-User

• Field name: Sec-Fetch-User
• Status: Permanent
• Structured Type: Item
• Specification document(s): https://www.w3.org/TR/fetch-metadata/#sec-fetch-user-header
• Comments:

[martinthomson]

These are all really useful -- in the sense that they are security-critical -- so I don't see why these wouldn't be registered.

As for the structured field syntax on each, these are all Items.

Mike, you have two Sec-Fetch-Dest entries (though the second has the right Field name in the template).

[mikewest]

These are all really useful -- in the sense that they are security-critical -- so I don't see why these wouldn't be registered.

As for the structured field syntax on each, these are all Items.

And I now notice I need to update the reference to the latest RFC. I'll take care of that shortly.

Mike, you have two Sec-Fetch-Dest entries (though the second has the right Field name in the template).

Thanks! Fixed.

[mozfreddyb]

Is Sec-Fetch-Ancestors going to come quickly enough that they should be included?

[mikewest]

Is Sec-Fetch-Ancestors going to come quickly enough that they should be included?

Given that these four are shipping ~everywhere, it seems reasonable to proceed with their "permanent" registration. Given the state of the discussion/implementation around the ancestors proposal, I'd expect its registration to be "provisional" and probably worth splitting out. It's not actually clear to me that it's going to be one header, for instance. :)