Absolute sizing dimensions should not be utilised to specify font size. When absolute font sizes are used, users may not be able to increase or decrease the size of the font or the font may become unreadable or overlap other content. Declaring font sizes in relative units will not only allow the user to increase or decrease the font size but will also automatically display in a size relative to their existing browser settings.
Text font size specified in em units allows user agents to scale content effectively. Since the em is a property of the font, it scales as the font changes size. If a font-size is specified for the body element, all other elements inherit that value, unless overridden by a more specific selector.
Absolute font size:
Accessibility (WCAG 2 Level AA, 1.4.4)
The checkpoint examines the entire HTML source code of a page and reports if any absolute font size are used within the measurement tags.