Actually the 120% are relative to the font size of a parent element with font size, in this case the (default) body font size.
You can’t specify a size relative to a default font size of an element unless it’s a child thereof. So, h1.bigger would only be 120% of the usual h1 if it was itself a child of h1. But that’s not valid HTML. So, you’ll always have to specify those sizes as absolute ones (or, relative to the body or any parent element with font size, respectively), when compared to each other.
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