JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="one" class="month-wrapper">
    <li>Test</li>
</ul>
<ul class="year-wrapper">
    <li>Test</li>
</ul>

CSS

ul.month-wrapper li
{
    font-size: 13px;
    background-color: #0FF;
}
ul.year-wrapper li
{
    font-size: 16px;
    background-color: #FF0;
}

JavaScript

var MonthWidth = 30;
var YearWidth = MonthWidth * 12;

var ss = document.styleSheets[0];
if(ss.insertRule){
   $('#one').css('width', 20 + 'px');
}