2010 ADA Standards for Accessible Design Pocket Guide
707.6.3 Function Keys.
Function keys shall comply with 707.6.3.
Function keys shall contrast visually from background surfaces. Characters and symbols on key surfaces shall contrast visually from key surfaces. Visual contrast shall be either light-on-dark or dark-on-light.
EXCEPTION: Tactile symbols required by 707.6.3.2 shall not be required to comply with 707.6.3.1.
Function key surfaces shall have tactile symbols as follows: Enter or Proceed key: raised circle; Clear or Correct key: raised left arrow; Cancel key: raised letter ex; Add Value key: raised plus sign; Decrease Value key: raised minus sign.