[PuerkitoBio]

It is ambiguous only if you think it can be a completely useless format. If you decide to use it, chances are you considered it useful, and the only useful way this can parse is as a number, otherwise it would't be able to represent numbers, booleans and null. I'd say it parses pretty much as expected, and as such, contrary to other comments, you don't have to go back to the docs every so often.