WordPress default theme css units

Is there a specific rule the default WordPress themes follow for CSS units?

I see em’s on some font sizes and rems on others and px on certain places.

Referred the following links but I couldn’t find the actual reference to the rule they use for their CSS units:
why does twenty eleven theme mix up the css measurement units?
Why do WordPress default themes specify fonts in so many places?

Edit: By rule, I meant, for eg: Always follow the unit system of – px for borders, em for spacing etc

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