If a font-family is not working that are used in HTML page and CSS, you need to import by Google fonts or with @font-face.
In Google fonts you can use import by HTML or CSS:
On your HTML:
<link href='https://fonts.googleapis.com/css?family=Open+Sans' rel='stylesheet' type='text/css'>On your CSS:
@import url(https://fonts.googleapis.com/css?family=Open+Sans);