<!--
  SVG
  https://codepen.io/erikdkennedy/pen/ygpwZg?editors=1100
-->
<header>
  <h1>Header Content</h1>
  <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" preserveAspectRatio="none">
    <polygon fill="red" points="0,100 100,0 100,100"/>
  </svg>
</header>

<section>
  <h1>Section Content</h1>
</section>