Chapter 3. Units and Values em and ex units
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<H1>Left margin = 24 pixels</H1>
<H2>Left margin = 18 pixels</H1>
<P>Left margin = 12 pixels</P>
<P>Although this text is the default size for the user agent, the
<SMALL>small-text element</SMALL> within the paragraph has a font
size which is 80% the rest of the text in the paragraph.</P>