image.png

  1. <!DOCTYPE html>
  2. <html lang="en">
  4. <head>
  5.   <meta charset="UTF-8">
  6.   <meta http-equiv="X-UA-Compatible" content="IE=edge">
  7.   <meta name="viewport" content="width=device-width, initial-scale=1.0">
  8.   <title>Document</title>
  10.   <style>
  11.     .box1 {
  12.       width: 0;
  13.       height: 0;
  14.       border-top: 10px solid blue;
  15.       border-right: 10px solid red;
  16.       border-bottom: 10px solid green;
  17.       border-left: 10px solid skyblue;
  18.       margin: 10px auto;
  19.     }
  20.   </style>
  21. </head>
  23. <body>
  24.   <div class="box1"></div>
  25. </body>
  27. </html>

效果图
image.png
所以另外三个都为透明色，看到的有颜色的就是三角。
也就是先全部为透明色，在令其中一个有颜色。

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

  <style>
    .box2 {
      width: 0;
      height: 0;
      border: 10px solid transparent;
      border-left-color: blue;
      margin: 10px auto;
    }
  </style>
</head>

<body>
  <div class="box2"></div>
</body>

</html>

效果图
image.png
为了兼容性更好，加上image.png

    .box2 {
      width: 0;
      height: 0;
        line-height: 0;
        font-size: 0;
      border: 10px solid transparent;
      border-left-color: blue;
      margin: 10px auto;
    }

三角应用-京东效果

image.png
image.png
代码

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

  <style>
    * {
      margin: 0;
      padding: 0;
    }

    .jd {
      position: relative;
      width: 100px;
      height: 200px;
      background-color: blue;
      margin: 50px auto;
    }

    .jd span {
      position: absolute;
      top: -19px;
      right: 20px;
      width: 0;
      height: 0;
      line-height: 0;
      font-size: 0;
      border: 10px solid transparent;
      border-bottom-color: blue;
    }
  </style>
</head>

<body>
  <div class="jd">
    <span></span>
  </div>
</body>

</html>

image.png