JSFiddle - React, Tailwind, and code Playground

HTML

<div class="flex-container">
  <div class="flex-item">flex item 1</div>
  <div class="flex-item">flex item 2</div>
  <div class="flex-item">flex item 3</div>
</div>

CSS

html, body {
    width: 100%;
    /* height: 100%; */
    }
    body {
        margin: 0;
        padding: 0;
        }
* { margin: 0; padding: 0; }

*,
*:before,
*:after {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
    }
.flex-container {
    margin: 0;
    }
.flex-container,
.flex-container * {
    margin: 0;
    }

.flex-container {
  display: -webkit-flex;
  display: flex;
  width: 400px;
  height: 100%;
  background-color: lightgrey;
}
.flex-item {
  background-color: cornflowerblue;
  width: 100px;
  height: 100px;
  margin: 10px;
}