使用css完成一个漂亮的信封边框

1.html部分：

<div class="letter-box">
  <div class="letter-border">
  </div>
</div>

2.css部分：

.letter-border {
    width: 100%;
    height: 100%;
    background: #fbfaf5;
}

.letter-box {
    width: 278px;
    height: 176px;
    padding: 5px;
    box-sizing: border-box;
    background: linear-gradient(45deg, #f25953 12.5%, #fbfaf5 12.5%, #fbfaf5 25%, #5590d6 25%, #5590d6 37.5%, #fbfaf5 37.5%, #fbfaf5 50%, #f25953 50%, #f25953 62.5%, #fbfaf5 62.5%, #fbfaf5 75%, #5590d6 75%, #5590d6 87.5%, #fbfaf5 87.5%, #fbfaf5 100%);
    background-size: 70px 70px;
}

效果：