禁止用户选中东西

elem.setAttribute("unselectable" , "on");
elem.style.MozUserSelect = 'none';
elem.style.WebkitUserSelect =? 'none';

?

elem.setAttribute('unselectable' , 'off');
elem.style.MozUserSelect = '';
elem.style.WebkitUserSelect = '';

?

?

ie 和 opera 用 unselectable on

firefox用 MozUserSelect none

safari,chrome用WebkitUserSelect none