The Font used in RuneScape Classic is a bitmap variant of Arial without anti-aliasing. The game uses bold variants of font sizes 12, 13, 14, 16, 20 and 24, as well as regular variants for sizes 11 and 12.
For most of the game interface, size 12 bold Arial is used. However, for certain components that need to be small and unassuming, such as the chat tabs, size 11 regular Arial is used instead.