A pseudo-element is not part of the DOM. Therefore, you cannot change its CSS properties directly through JS.
In order to get your desired effect the way you want it, my best guess would be YUI.StyleSheet and manipulate the stylesheet itself, although I have to admit I haven't tested it myself in recent years.
Including such a utility and doing all of this calculation seems like a lot of work for width matching.
If you are willing to compromise a little bit on the semantic HTML, there is a working technique:
Your element takes the entire width of the screen. Wrapping the text with a span and adding the pseudo-element to that, as an inline-block should allow you to get the border under the text only
HTML:
<h1 class="title"><span>Hello World</span></h1>
CSS:
.title {
border-bottom: 3px solid #aaa;
position: relative;
}
.title span{
display:inline-block;
position:relative;
}
.title span:after {
content: "";
width: 100%;
border-bottom: 3px solid red;
display: block;
position: absolute;
}
Here is my version of the codePen.
For future reference:
There is a W3C Candidate Recommendation that suggests the capability of using attributes for CSS properties other than content.
This way, if and when the recommendation is approved and implemented, it might be possible to have the pseudo-element reflect the parent's attributes, as such:
this.setAttribute("length", $(this).textWidth());
And the relevant CSS:
.title:after {
...
width: attr(length px);
...
}
