user agent - Why does Chrome use sec-ch-ua: ""Not\A;Brand";v="99"?

Question

I understand that making user-agent hints more ambiguous is intended, in part, to make browser fingerprinting harder.

My own (Windows desktop) Chrome sends the headers:

user-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.75 Safari/537.36
sec-ch-ua: "Chromium";v="86", ""Not\A;Brand";v="99", "Google Chrome";v="86"
sec-ch-ua-mobile: ?0

What I don't get is:

1. Why the string "Not A Brand" specifically? Does anyone else use this pseudo-UA? Is this a joke of some sort?
2. Why the " and \A; inside the string? My only guess is that this is supposed to mess with parsers somehow (like the anti-IE hacks in CSS), but that seems like a rather odd purpose — and IIRC, A is the bell character.
3. How is this supposed to accomplish user-agent hint ambiguity, given that it also sends the full user-agent header, which has the specific version numbers?
4. While at it: why does Chrome's user-agent also claim to be Mozilla, AppleWebKit, and Safari? It isn't, and this user-agent string is distinctively Chrome's. Does it have some sort of embedded components from those other browsers?

question from:https://stackoverflow.com/questions/64413275/why-does-chrome-use-sec-ch-ua-not-abrandv-99

Answer 1

It seems that it's part of Chromium's GREASEing strategy:

User agents' brands containing more than a single entry could encourage standardized processing of the UA string. By randomly including additional, intentionally incorrect, comma-separated entries with arbitrary ordering, they would reduce the chance that we ossify on a few required strings.

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Looking at the Chromium repository, it seems that it was introduced in this commit

The commit description given is:

[client-hints] GREASEing the Sec-CH-UA list

Randomizing order and string with escaped characters to ensure proper
parsing and prevent ossification.

It also links to this ticket in the bug tracker.