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The font size of display characters is expressed in relative size and not absolute size.
Allow users to enlarge fonts that are displayed on the screen.
Don’t modify the default font size. Use a unit which is relative to the user’s settings (em, ex, %) and not a unit which is absolute and dependent on the rendering of the device being used to access the information (px, pt).
Read this link : http://diveintoaccessibility.org/day_26_using_relative_font_sizes.html
Test the page by changing the font size settings in all of the main web navigators.