To create a triangle using CSS, we can use the border property to set the width, height, and color of the triangle, and then use the border-style property to create a triangle shape. Here's an example of how to do it: HTML:

  <div class="triangle"></div>


.triangle {
    width: 0;
    height: 0;
    border-left: 30px solid transparent;
    border-right: 30px solid transparent;
    border-bottom: 30px solid red;

In this example, we've created a div element with a class of triangle. We've set the width and height of the element to 0, which effectively hides the element. We've then used the border-left, border-right, and border-bottom properties to create the triangle shape