Responsive Web Design - Use Legible Font Sizes

1. Configure the viewport to make sure fonts will be scaled as expected across various devices. 
2. Use a base font size of 16 CSS pixels. Adjust the size as needed based on the font used.
3. Use sizes relative to the base size to define the typographic scale.
4. Text needs vertical space between its characters and may need to be adjusted for each font. The general recommendation is to use the browser default line-height of 1.2em.
5. Restrict the number of fonts used and the typographic scale: too many fonts and font sizes lead to messy and overly complex page layouts.

read more here: https://developers.google.com/speed/docs/insights/UseLegibleFontSizes