https://css-tricks.com/snippets/css/css-triangle/

CSS

.arrow-up {
  width: 0; 
  height: 0; 
  border-left: 5px solid transparent;
  border-right: 5px solid transparent;
  
  border-bottom: 5px solid black;
}

.arrow-down {
  width: 0; 
  height: 0; 
  border-left: 20px solid transparent;
  border-right: 20px solid transparent;
  
  border-top: 20px solid #f00;
}

.arrow-right {
  width: 0; 
  height: 0; 
  border-top: 60px solid transparent;
  border-bottom: 60px solid transparent;
  
  border-left: 60px solid green;
}

.arrow-left {
  width: 0; 
  height: 0; 
  border-top: 10px solid transparent;
  border-bottom: 10px solid transparent; 
  
  border-right:10px solid blue; 
}

HTML

<div class="arrow-up"></div>
<div class="arrow-down"></div>
<div class="arrow-left"></div>
<div class="arrow-right"></div>