How to draw a triangle with CSS

.block {
    display: inline-block;
    height: 100px;
    width: 100px;
    background: blue;
    /* polygon 定义多边形， ()里面的每对参数表示多边形的顶点坐标（X,Y），也就是连接点 */
    clip-path: polygon(0 0, 100% 0, 0 100%, 0 0);
}

clip-path: polygon(0 0, 100% 0, 0 100%, 0 0);

Note: the upper left corner is 0 0 point