<header>
  <h1>Header Content</h1>
  <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" preserveAspectRatio="none">
    <polygon fill="white" points="0,100 100,0 100,100"/>
  </svg>
</header>

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

header {
  position: relative;
  height: 300px;
  background-image: linear-gradient(#ff9d2f, #ff6126);
}

h1 {
  padding: 100px 0;
  font: 44px "Arial";
  text-align: center;
}

header h1 {
  color: white;
}

svg {
  position: absolute;
  bottom: 0;
  width: 100%;
  height: 10vw;
  /* set height to pixels if you want angle to change with screen width */
}