The font size can be changed in almost all browsers so:
Testet in Windows 8, Windows 7, Windows XP with Internet Explorer and Firefox under Linux Mint 17 with Firefox (respectively the latest version).
As with the keyboard:
Hold down the Ctrl key and the + or - button (standard size: Ctrl key and the number 0).
With keyboard and mouse:
Hold Ctrl key and scrolling.
With the menu bar of the browser:
View> Zoom (If the menu bar is missing, press F10)
Apple Safari-Browser:
holding Apple key and the + or - button.
or
Menu Bar: view:
or
Adjust the minimum font size in the menu:
Safari > Settings > Tab: Advanced