HTML Multiselect Limit

Answer 1

Script that will disallow more than 3 elements to be selected (as opposed to just validating it at submit time).

http://jsfiddle.net/v33sszgp/

var verified = [];
document.querySelector('select').onchange = function(e) {
  if (this.querySelectorAll('option:checked').length <= 3) {
      verified = Array.apply(null, this.querySelectorAll('option:checked'));
  } else {
    Array.apply(null, this.querySelectorAll('option')).forEach(function(e) {
        e.selected = verified.indexOf(e) > -1;
    });
  }
}

Note there's some additional complexity related to preventing the item from actually being selected on the select and that it rather validates a user's actions and reverts it if it's invalid.