Most of the things you've listed here don't actually seem all that reasonable to me.

User agents as a concept are rather poorly thought out across the board and not all that useful but persist because that's just how technical cruft is.

Fonts should be provided by the website; if not provided the choice should take the form of a spec sent by the website including line height, sarifs or not, monospace or not, etc. There's little to no excuse for the current font situation IMO beyond poor design decisions that became heavily entrenched.

Timezone and other obviously private metadata should never be shared without the user explicitly granting permission on a case by case basis. The status quo here is completely inexcusable as is the continued failure to fix the problem.

Size of the physical screen should never be exposed under any circumstances. The current size of the browser window is reasonable on its face but now that fingerprinting is understood to be an issue should always be heavily letterboxed unless the user consents to sharing the exact value.

Video formats should be provided by the website as a list of offerings and the browser should respond with a choice; the user could optionally intervene. There's no reason to expose the full capabilities to a remote service.

Querying the current time should be gated behind an explicit permission. There's almost never a need for it. However from a fingerprinting perspective you also have to worry about correlating the rate of clock skew across clients. That can be solved by gating access to high resolution time counters behind an explicit permission as (once again) the vast majority of services have no legitimate use for such functionality.

All of these could have a set of standard non identifiable answers (eg. firefox reports the same 20 fonts, couple video formats, one among a few standard window sizes etc.) and for anything more extensive/precise, it would require the user's authorization and the user should have the option of feeding fake info (eg. fake timezone)

The tor project seeks this bypass this by keeping such things standardized across users, even down to reported screen size. And there is nothing stopping the browser from fibbing as most settings dong matter all that much (ie UK v Canadian v American English).

I fantasize having a browser that I can use only for viewing content.

No applications. No mail. No need for cookies.

I can use a "regular" browser for more enhanced stuff. But for simple content consumption, we can just have a "dumb" browser that can't do much.

> A user agent that says the browser's version? Reasonable enough.

No user agent. I'm guessing it will need it for JavaScript or HTML features, and dynamically update if using an old browser, but let's just not supply a user agent and let it be the reader's burden to have a reasonably decent browser.

> Being able to ask for fonts, if the system has them? Difficult to have font support without that.

What's the fallback if the system doesn't have them?

> Getting the user's timezone, language and keyboard layout? Reasonable.

Keyboard layout is irrelevant for viewing content. For timezone and language: Yeah, I can see the use cases, but these are in a small minority. Let there be a popup when requested, and the user can specify the timezone/language as requested.

> The size of the screen, and the size of the browser window? Difficult to lay things out without that.

Let's let this new browser return only from a (small) discrete set of sizes. It will pick the size closest to the actual browser window size and send that.

> Of course a video or audio player needs to know which video formats your browser supports - how else to provide the right video?

Same answer as user agent. Either let the user pick from a selection of video formats, or just hard code a reasonable one and put the onus on the user to have a browser that supports it.

> Obviously javascript can get the time, and it's trivial to figure out the system's clock error by comparing that to the time on a server.

This hypothetical browser could just not send the time :-) For 99% of content consumption, this function is not needed.

What I'm describing should be part of "Private mode". Or browsers should have an "Ultra-private" mode that is the above. If it's too complex/risky maintaining it all in one codebase ... fine. Just have a separate browser.

Right now, if I built such a browser, I'm sure a lot of sites meant for content would break. But in my fantasy world, using "Ultra-private" would be the default, and people who make sites will target them first.

I think much of the complexity in making a web browser is all the "other" stuff. Being able to run apps, cookie/privacy management, etc.

most people would expect something different from tor, surely.

Most stock android phones don't either. You usually get to control precise location, notifications, some background activity, SMS, Calls, Mic, Camera, SD Card, etc.

But most ROMs don't allow controls for WiFi, Cell data, Phone ID, Phone number, User ID, local storage, etc...