CodePen

HTML

            
              
<div class="triangle topleft"></div>
<div class="triangle topright"></div>
            
          
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↑ Insert the most common viewport meta tag

CSS

            
              
.triangle{width:0px;margin:auto;}
.topleft{border-top:150px solid blue;border-right:150px solid transparent;}
.topright{border-bottom:150px solid blue;border-left:150px solid transparent;margin-top:-150px;}
            
          
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JS

            
              
            
          
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