Could you please explain me or send me a link where is explained how does CSS property font-size work?
When you're working with relatives sizes in %. It's the percent of what? What are best practices etc.
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The best place to go is where CSS is defined - http://www.w3.org/TR/CSS2/fonts.html#font-shorthand. To answer the % question - it's the percentage of the font size of the parent element.