纯css 四边形的角样式（只有两个角是三角，其他两个是线段）

效果如图：

核心：使用伪类

代码如下： <div class="box-style"></div>

.box-style {
  position: relative;
  //纯css只有四个角有边框的样式
  box-shadow: 0px 0px 12px 1px #003ba26b inset;
  background: linear-gradient(to left, #1f5fd3, #1f5fd3) left top no-repeat,
    linear-gradient(to bottom, #1f5fd3, #1f5fd3) left top no-repeat,
    linear-gradient(to left, #1f5fd3, #1f5fd3) right top no-repeat,
    linear-gradient(to bottom, #1f5fd3, #1f5fd3) right top no-repeat,
    linear-gradient(to left, #1f5fd3, #1f5fd3) left bottom no-repeat,
    linear-gradient(to bottom, #1f5fd3, #1f5fd3) left bottom no-repeat,
    linear-gradient(to left, #1f5fd3, #1f5fd3) right bottom no-repeat,
    linear-gradient(to left, #1f5fd3, #1f5fd3) right bottom no-repeat;
  background-size: 1px 11px, 11px 1px, 1px 11px, 11px 1px;
}

.box-style:before {
  content: '';
  width: 0;
  height: 0;
  position: absolute;
  left: 0px;
  top: 5px;
  display: inline-block;
  border: 6px solid transparent;
  border-left-color: #003596;
  transform: rotate(225deg) translate3d(-1px, 8px, 0);
  transform-origin: 0%;
}

.box-style:after {
  content: '';
  width: 0;
  height: 0;
  position: absolute;
  right: -20px;
  top: -2px;
  display: inline-block;
  border: 6px solid transparent;
  border-right-color: #003596;
  transform: rotate(138deg) translate3d(-1px, 8px, 0);
  transform-origin: 0%;
}