CSS FONT-SIZE

You can manipulate the size of your fonts by using values, percentages, or key terms. Using values are useful if you do not want the user to be able to increase the size of the font because your site will look incorrect if they did so. Percentages are great when you want to change the default font, but do not want to set a static value

Example
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body {
  font-size: x-large
}
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p {
  font-size: 10px
}

Possible Values

xx-small, x-small, small, medium, large, x-large, xx-large: Sets the size of the font to different sizes, from xx-small to xx-large
smaller: Sets the font-size to a smaller size than the parent element
larger: Sets the font-size to a larger size than the parent element
length: Sets the font-size to a fixed size
%: Sets the font-size to a % of  the parent element