XO infers rules from prettier in a way that I don't think is the best. From readme:

prettier

[...]

[...] and if not set will be determined as follow:

• semi: based on semicolon option
• useTabs: based on space option
• tabWidth: based on space option
• trailingComma: none
• singleQuote: true
• bracketSpacing: false
• jsxBracketSameLine: false

I think that trailingComma, singleQuote, bracketSpacing and jsxBracketSameLine should pick the same defaults from Prettier instead of having another predefined default choice made by xo.

The current behavior caused some confusion to me and someone else in #440.