JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main-conteiner">
    <div class="block1">First</div>
    <div class="block2">Second</div>
</div>

CSS

main-conteiner {
  width: 600px;
  margin: 0 auto;
}

.block1 {
  width: 50%;
  float:left;
  background: red;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  -moz-box-sizing: border-box;
}

.block2 {
  width: 50%;
  float:left;
  background: yellow;
  -webkit-box-sizing: border-box;
  box-sizing: border-box;
  -moz-box-sizing: border-box;
}

div {
  margin: 0;
  padding: 0;
}