How can I use percents in `border-*` properties?

Question

I have code which uses the Twitter Bootstrap 3, nav with right-arrow, which I created using border-* properties. But if I use very long text in right-arrow, it does not expand, and if I use percents, the code will not work...

Example on JsFiddle

enter image description here

.custom-nav-stacked>li>a {
  border-radius: 2px;
}

.custom-nav-stacked>li.active>a:after,
.custom-nav-stacked>li.active>a:hover:after,
.custom-nav-stacked>li.active>a:focus:after {
  content: '';
  position: absolute;
  left: 100%;
  top: 50%;
  margin-top: -19px;
  /*margin-left: -1px;*/
  border-top: 19px solid transparent;
  border-left: 13px solid #428bca;
  border-bottom: 19px solid transparent;
}

<div class="container" style="margin-top:  20px">
  <div class="row">
    <div class="col-md-2" style="width: 300px /* width there only for pretty demo */">
      <ul class="nav nav-pills nav-stacked custom-nav-stacked">
        <li class="active"><a href="#">A long long text</a></li>
        <li><a href="#">Small text</a></li>
        <li class="active"><a href="#">A long long long long long long long long long text</a></li>
        <li><a href="#">Small text 111</a></li>
      </ul>
    </div>
  </div>
</div>

Example on JsFiddle

How can I make the triangle responsive based on the amount of text?

duplicate off http://stackoverflow.com/questions/13474754/how-to-set-borders-thickness-in-percentages — Danny van Holten, Jul 30 '14 at 23:55
Side note: the `a:hover` and `a:focus` using the same CSS is not needed as the C in CSS stands for *Cascading*, which means the styles will apply to all states of the element by default — Zach Saucier, Jul 31 '14 at 02:09

score 6 · Accepted Answer · edited May 23 '17 at 10:30

The only relative unit (meaning reactive to something else) that border can use is the vh and vw units. As a result, border can only be responsively sized when the element it is on is also sized relative to the viewport. Demo

As a result, what you're trying to do with CSS is not currently possible because if you set the height and border with viewport units then they will not respond to the text content. You'd have to give a class to the ones of varying height, thus defeat the purpose of relative sizing anyway.

However, this is pretty easy to do using javascript. You just need to iterate through the relevant elements, calculate the height of the element, divide that by 2 and make that the border-top and border-bottom, then make a proportion of that the border-left. Demo of that

/* JS */
var actives = document.getElementsByClassName("active"),
    triangles = document.getElementsByClassName("triangle");

for(var i = 0, l = actives.length; i < l; i++) {
    triangles[i].style.borderTopWidth = actives[i].clientHeight / 2 + "px";
    triangles[i].style.borderBottomWidth = actives[i].clientHeight / 2 + "px";
    triangles[i].style.borderLeftWidth = actives[i].clientHeight / 3 + "px";
    triangles[i].style.marginTop = - actives[i].clientHeight / 2 + "px";
}

/* CSS */
li.active .triangle {
    position: absolute;
    left: 100%; /* Position it to the right */
    top: 50%;
    border-color:transparent; /* Make all other sides transparent */
    border-left-color:#428bca; /* Add color to the side that matters */
    border-style:solid; /* This property is necessary to make it show */
}

Actual (meaning in the DOM) elements are suggested as opposed to pseudo elements using the javascript approach because they are much easier to access using the DOM. If you do end up using pseudo elements, you will need to change the actual stylesheets which is more difficult

How can I use percents in `border-*` properties?

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