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

  • I am not sure there is any specific "rule" being followed Commented Sep 16, 2017 at 5:27
  • So its all just random? Pardon me, I'm fairly new to WP styling.
    – AashikP
    Commented Sep 16, 2017 at 5:54
  • would not call it "random", I assume it is based on the experience which the theme developer(s) has and what kind of feedback they receive. Keep in mind that CSS best practices keep changing from time to time. Commented Sep 16, 2017 at 5:59

1 Answer 1


I would suggest looking into the ways that the various units function. There would rarely be a case where you would use one unit exclusively. I generally use ems, rems, and %.

This article may help get you started: understanding and using rem units in css

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