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The font size in html/css is set relative if you use either em or percent.
1em = 100%
2em = 200%
The value always changes the font size given by the parent.
If no font size is given, the font size of the parents parent is used.
Somewhere in either your css or the cs f the reading software a font size is given in px or pt.
That is font size all relative font sizes are based on.
In your example let say the base font size of your epub is 14pt.
In h1 you change it to 2em = 200%. The font size of the heading 1 becomes 28pt (14*2).
The font size of .subtitle is not changed so it inherits the font size of its parent the h1 class.
The class .subsubtitle has a set font size of 1em = 100% which means 1 time the font size of its parent h1 (28 * 1). To make the font size smaller, you will have to use an em value smaller that 1.
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