//Specify affected tags. Add or remove from list:
var tgs = new Array('span','li','h2','h3','h4');

var min=11;
var max=13;
function ts() {

	for ( i = 0 ; i < tgs.length ; i++ ) {

		var cTags = document.getElementsByTagName(tgs[i]);

		for ( j = 0 ; j < cTags.length ; j++ ) {
              if(cTags[j].style.fontSize) {
                var sz = parseInt(cTags[j].style.fontSize.replace('px',''));
                if (sz <= min) {
                  sz = sz+2;
                } else {
                  sz = sz-2;
                };
              } else {
                var sz = 13;
              };

              cTags[j].style.fontSize=sz+'px';
            }
	}
}