How to Create a Multiple Selection List Box in JavaScript for HTML5 and CSS3 Programming - dummies
GitHub - PhilippeMarcMeyer/vanillaSelectBox: A dropdown menu with lots of features that takes a select tag and transforms it into a single or multi- select menu with 1 or 2 levels
Moving elements or shifting options from one drop down list box to other in JavaScript
HTML Select Tag – How to Make a Dropdown Menu or Combo List