Responsive CSS triangle with percents width

Question

The code below will create an arrow right below an <a> element:

JSFiddle

.btn {
    position: relative;
    display: inline-block;
    width: 100px;
    height: 50px;
    text-align: center;
    color: white;
    background: gray;
    line-height: 50px;
    text-decoration: none;
}
.btn:after {
    content: "";
    position: absolute;
    bottom: -10px;
    left: 0;
    width: 0;
    height: 0;
    border-width: 10px 50px 0 50px;
    border-style: solid;
    border-color: gray transparent transparent transparent;   
}

<a href="#" class="btn">Hello!</a>

The problem is that we have to indicate the link width to get an arrow of a proper size because we cannot indicate the border width in pixels.

How to make a responsive triangle percent based?

Answer 1

You could use a skewed and rotated pseudo element to create a responsive triangle under the link :

DEMO (resize the result window to see how it reacts)

The triangle maintains it's aspect ratio with the padding-bottom property.

If you want the shape to adapt it's size according to it's content, you can remove the width on the .btn class

.btn {
  position: relative;
  display: inline-block;
  height: 50px; width: 50%;
  text-align: center;
  color: white;
  background: gray;
  line-height: 50px;
  text-decoration: none;
  padding-bottom: 15%;
  background-clip: content-box;
  overflow: hidden;
}
.btn:after {
  content: "";
  position: absolute;
  top:50px;  left: 0;
  background-color: inherit;
  padding-bottom: 50%;
  width: 57.7%;
  z-index: -1;
  transform-origin: 0 0;
  transform: rotate(-30deg) skewX(30deg);
}
/** FOR THE DEMO **/

body {
  background: url('http://i.imgur.com/qi5FGET.jpg');
  background-size: cover;
}

<a href="#" class="btn">Hello!</a>

For more info on responsive triangles and how to make them, you can have a look at Triangles with transform rotate (simple and fancy responsive triangles)