利用边框写一个三角形

div {
            width: 40px;
            height: 40px;
            border-width: 40px;
            border-style: solid;
            border-color: red green blue brown;
        }

div {
            width: 0;
            height: 0;
            border-width: 40px;
            border-style: solid;
            border-color: red green blue brown;
        }

div {
            width: 0;
            height: 0;
            border-width: 40px;
            border-style: solid;
            border-color: red transparent transparent transparent;
        }