No-Library (pure JS), HTML, CSS, JavaScript
Credit: https://jsfiddle.net/Jan_Miksovsky/yy7Zs/
Credit: https://www.youtube.com/watch?v=wfMtDGfHWpA
Credit: https://www.codewars.com/kata/largest-5-digit-number-in-a-series/train/javascript
Credit: http://www.codewars.com/kata/valid-parentheses/train/javascript