CODE EXAMPLE FOR CSS

triangle css

/*corner triangles if you want normal triangles look at other answers*/
    #triangle-topleft {
      width: 0;
      height: 0;
      border-top: 100px solid red;
      border-right: 100px solid transparent;
    }
    #triangle-topright {
      width: 0;
      height: 0;
      border-top: 100px solid red;
      border-left: 100px solid transparent;
    }
    #triangle-bottomleft {
      width: 0;
      height: 0;
      border-bottom: 100px solid red;
      border-right: 100px solid transparent;
    }

    #triangle-topright {
      width: 0;
      height: 0;
      border-top: 100px solid red;
      border-left: 100px solid transparent;
    }