JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
  <head>
  </head>
  <body>
      <header id="sidebar">
         blablabla
      </header>
      <section id="main">
         <h1>Gotcha!</h1>
      </section>
  </body>
</html>

CSS

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

body { display: table }
 
          #sidebar {
            width:25%;
            border-right:1px solid #222;
            background-color: #E0E0E0;
          }

          #sidebar, #main {
            display:table-cell;
            vertical-align: top; /* optional */
          }

          h1 {
            padding:2%;
            margin-top: 0;
            /* margin:0; */
            /* defining the margin as zero aligns 
             * everything at the top */
          }