JSFiddle - React, Tailwind, and code Playground

HTML

<div class="simple">test1</div>
<div class="mod">test2</div><!-- it should be 100px -->

CSS

.simple{
    width:calc(300px - 100px);
    background:blue;
}

.mod{
    width:calc(100% / 4);
    background:red;
}

html{color:white;}