CSS Font Sizes - REMs with PX fallback for older broswers (snippet pulled from Base Framework: http://matthewhartman.github.io/base/)

LESS Mixin

// Font Sizes (REMs with PX fallback for older browsers)
.font-size (@sizeValue) {
  @remValue: @sizeValue / 16;
  @pxValue: (@sizeValue);
  font-size: ~"@{pxValue}px"; 
  font-size: ~"@{remValue}rem";
}

// Line Height Sizes (REMs with PX fallback for older browsers)
.line-height (@sizeValue) {
  @remValue: @sizeValue / 16;
  @pxValue: (@sizeValue);
  line-height: ~"@{pxValue}px"; 
  line-height: ~"@{remValue}rem";
}