如何在图像上方添加渐变边框（对角线边框）[重复]

我需要这样的东西

但在图像上方，这是我的第一次尝试，但它不起作用

http://jsfiddle.net/wo8gbhx3/17/ http://jsfiddle.net/wo8gbhx3/17/

这是我的标记（现在）

HTML

<div class="testing">
    <ul>
        <li class="selected unavailable">
            <a href="#">
                <img src="http://placehold.it/25x25"/>
            </a>
        </li>
    </ul>
</div>

CSS

.testing ul {
    list-style: none;
}
.testing ul li {
    width: 25px;
    height: 25px;
    position: relative;
}
.testing ul li img {
    width: 100%;
    height: 100%;
}
.unavailable:before {
    position: absolute;
    border: 2px solid green; /* Just for testing */
    background:repeating-linear-gradient( 150deg, #FFF, #FFF 16px, #000 18px);
}

你需要这样的东西

.testing ul {
  list-style: none;
}
.testing ul li {
  width: 250px;
  height: 250px;
}
.testing ul li img {
  width: 100%;
  height: 100%;
  display: block;
}
.unavailable {
  position: relative;
}
.unavailable a:after {
  content: '';
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  position: absolute;
  border: 2px solid green;
  /* Just for testing */
  background: repeating-linear-gradient(150deg, transparent, transparent 16px, #000 18px);
  z-index: 2;
}

<div class="testing">
  <ul>
    <li class="selected unavailable">
      <a href="#">
        <img src="http://placehold.it/200x200" />
      </a>
    </li>
  </ul>
</div>