Re: [w3c/browser-payment-api] How will the spec address versioning / feature detection? (#33)

@adrianba --
>The problem with having `supports()` methods is that they often don't provide the right granularity or web developers make mistakes that proliferate, which causes implementations to have to lie about support. We've seen this with such methods before.

I agree in the _general_ case, where the binding between feature names and specific function calls is only defined via specification, that this is true. When you're trying to figure out whether you can include a value in a dictionary and have it honored, using a dictionary with exact key names is 100% unambiguous. By contrast, what you're proposing -- using the presence of a member on a structure to infer the validity of a differently-name dictionary key on another -- actually has the same kind of problem that you're highlighting with `supports()` methods: developers have to make somewhat oblique inferences rather than having something that can be determined programmatically.

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