UAs SHOULD have more than a single brand value (Was: Stronger encouragement on set of UA strings)

[domenic]

https://github.com/WICG/ua-client-hints#should-the-ua-string-be-a-set indicates an idea of there being a set of user agents.

Although it is unclear why this would be a good idea (#21), if we assume it's desirable, I worry that the current design does not do enough to encourage web developers to treat the UA list as a set. Unless early browser implementations follow this optional part of the spec, I imagine we'll see a lot of parsing code for Sec-CH-UA which assumes there's only a single value. (See also #7 (comment).) This will effectively prevent any later browser from ever implementing the optional steps in the spec, because doing so would break server assumptions.

Additionally, the JS API always returns a single string, not a set.

If the intention is to encourage treating the UA as a set of possible UAs, then I think you need to make changes such as:

• Changing Sec-CH-UA to a structured header list of strings, instead of a structured header string.
• Potentially mandating, or at least strongly suggesting, that browsers always include >= 2 items in the list. (Especially early implementations.)
• Change the JS API so that getUserAgent() returns a promise for an array of NavigatorUADatas, or possibly change NavigatorUAData's platform and version members to be arrays instead.

[yoavweiss]

I believe this is (almost) closed by #46. #49 adds various explanations that are also related.

The only thing missing is a SHOULD for including more than a single item.