<div class="Block1">
  <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>
 
<div class="Block2">
  <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>
body {
  font: 16px/1.5 sans-serif;
  color: #666;
  background: #F4F4F4;
  margin:0;
}
.Block1 {
  color: #1a1a1a;
}
p {
  width: 60%;
  margin: 0 auto;
  padding: 30px 0;
}

.Block2 {
  /*レイアウト用のCSS*/
  padding: 10px 0 40px 0;
  background-color: #191970;
  color: #ccc;
}
.Block1 {
  position: relative;
  padding: 20px 0 30px 0;
  
}
.Block1::before{
  content:"";
  position: absolute;
  bottom: 0;
  left: 0;
  width: 0;
  height: 0;
  border-style: solid;
 
  /*ここで三角形のサイズを決める。
  必ず幅100vwにして、ブラウザサイズいっぱいにしてください。*/
  border-width: 40px 0 0 100vw ;
 
  border-color: transparent  transparent  transparent #191970;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.