Users should be able to read and understand the information presented by your application. Your application should also be flexible enough to meet your user's reading needs. Check in particular:
Can the page be resized to 200% in the browser and still be usable?
Are text sizes appropriate across variable device widths?
Can the page still be read if the user is overriding your fonts with their own? (Try testing this using the Chrome addon OpenDyslexic)
Does the text need to be adjusted for colour or weight to provide adequate contrast? (Try your most used fonts in a contrast checker)