Services using AF do not resize text

[Shabana Ali]

Increasing the font size on Chrome, Firefox or IE11 does not increase the size of the text.

• Chrome: chrome://settings/fonts
• Firefox: about:preferences (although can achieve this by ticking "Zoom text only" and zooming)

GOV.UK Design System specifies font size in "em" and "px" as fallback. AF only has "px".

Reference hmrc/accessibility#43