JSFiddle - React, Tailwind, and code Playground

HTML

<main>
<div class="caja90">
  <h2>
    Titulo al 90%
  </h2>
</div>
<div class="caja100">
  <h2>
    Titulo al 100%
  </h2>
</div>
</main>

CSS

*{
  margin:0;padding:0;
}
main{
  width:100%;
  display:block;
}
.caja90{
  width:90%;
  background:green;
  margin:0 auto;
  height:80vh;
}
.caja100{
  width:100%;
  background:red;
  height:80vh;
}