Any way to declare a size/partial border to a box?

Not really. But it's very easy to achieve the effect in a way that degrades gracefully and requires no superfluous markup:

div {
  width: 350px;
  height: 100px;
  background: lightgray;
  position: relative;
  margin: 20px;
}

div:after {
  content: '';
  width: 60px;
  height: 4px;
  background: gray;
  position: absolute;
  bottom: -4px;
}
<div></div>

I know, this is already solved and pixels were requested. However, I just wanted to share something...

Partly underlined text elements can easily achieved by using display:table or display:inline-block

(I just don't use display:inline-block because, yeah you know, the awkward 4px-gap).

Textual Elements

h1 {
  border-bottom: 1px solid #f00;
  display: table;
}
<h1>Foo is not equal to bar</h1>

Centering, display:table makes it impossible to center the element with text-align:center.
Let's work around with margin:auto...

h1 {
  border-bottom: 1px solid #f00;
  display: table;
  margin-left: auto;
  margin-right: auto;
}
<h1>Foo is not equal to bar</h1>

Well, that's nice, but it's not partially.
As bookcasey already introduced, pseudo-elements are worth gold.

h1 {
  display: table;
  margin-left: auto;
  margin-right: auto;
}

h1:after {
  border-bottom: 1px solid #f00;
  content: '';
  display: block;
  width: 50%;
}
<h1>Foo is not equal to bar</h1>

Offset, the underline is left aligned right now. To center it, just push the pseudo-element the half of its width (50% / 2 = 25%) to the right.

h1 {
  display: table;
  margin-left: auto;
  margin-right: auto;
}

h1:after {
  border-bottom: 1px solid #f00;
  content: '';
  display: block;
  margin-left: 25%;
  width: 50%;
}
<h1>Foo is not equal to bar</h1>

...as davidmatas commented, using margin:auto is sometimes more practical, than calculating the margin-offset by hand.

So, we can align the underline to the left, right or center (without knowing the current width) by using one of these combinations:

  • Left: margin-right: auto (or just leave it off)
  • Middle: margin: auto
  • Right: margin-left: auto

Full example

.underline {
  display: table;
  margin-left: auto;
  margin-right: auto;
}

.underline:after {
  border-bottom: 1px solid #f00;
  content: '';
  display: block;
  width: 50%;
}

.underline--left:after {
  margin-right: auto; /* ...or just leave it off */
}

.underline--center:after {
  margin-left: auto;
  margin-right: auto;
}

.underline--right:after {
  margin-left: auto
}
<h1 class="underline underline--left">Foo is not equal to bar</h1>
<h1 class="underline underline--center">Foo is not equal to bar</h1>
<h1 class="underline underline--right">Foo is not equal to bar</h1>

Block-Level Elements

This can easily be adopted, so that we can use block-level elements. The trick is to set the pseudo-elements height to the same height as its real element (simply height:100%):

div {
  background-color: #eee;
  display: table;
  height: 100px;
  width: 350px;
}
div:after {
  border-bottom: 3px solid #666;
  content: '';
  display: block;
  height: 100%;
  width: 60px;
}
<div></div>

Tags:

Css

Border

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