30秒学会 CSS 片段 – Border with top triangle

Creates a text container with a triangle at the top.

• Use the :before and :after pseudo-elements to create two triangles.
• The color of the :before triangle should be the same as the container’s border color.
• The color of the :after triangle should be the same as the container’s background color.
• The border width of the :before triangle should be 1px wider than the :after triangle, in order to act as the border.
• The :after triangle should be 1px to the right of the :before triangle to allow for its left border to be shown.

预览

HTML

<div class="container">
  Border with top triangle
</div>

CSS

.container {
  position: relative;
  background: #ffffff;
  padding: 15px;
  border: 1px solid #dddddd;
  margin-top: 20px;
}

.container:before, .container:after {
  content: '';
  position: absolute;
  bottom: 100%;
  left: 19px;
  border: 11px solid transparent;
  border-bottom-color: #dddddd;
}

.container:after {
  left: 20px;
  border: 10px solid transparent;
  border-bottom-color: #ffffff;
}

翻译自：https://www.30secondsofcode.org/css/s/border-with-top-triangle