outline on only one border

Solution 1:

You can use box-shadow to create an outline on one side. Like outline, box-shadow does not change the size of the box model.

This puts a line on top:

box-shadow: 0 -1px 0 #000;

I made a jsFiddle where you can check it out in action.

The syntax is box-shadow: offset-x | offset-y | blur-radius | color

INSET

For an inset border, use the inset keyword. This puts an inset line on top:

box-shadow: 0 1px 0 #000 inset;

Multiple lines can be added using comma-separated statements. This puts an inset line on the top and the left:

box-shadow: 0 1px 0 #000 inset,
            1px 0 0 #000 inset;

For more details on how box-shadow works, check out the MDN page.

Solution 2:

Outline indeed does apply to the whole element.

Now that I see your image, here's how to achieve it.

.element {
  padding: 5px 0;
  background: #CCC;
}
.element:before {
  content: "\a0";
  display: block;
  padding: 2px 0;
  line-height: 1px;
  border-top: 1px dashed #000; 
}
.element p {
  padding: 0 10px;
}

<div class="element">
  <p>Some content comes here...</p>
</div>

(Or see external demo.)

All sizes and colors are just placeholders, you can change it to match the exact desired result.

Important note: .element must have display:block; (default for a div) for this to work. If it's not the case, please provide your full code, so that i can elaborate a specific answer.

Solution 3:

Try with Shadow( Like border ) + Border

border-bottom: 5px solid #fff;
box-shadow: 0 5px 0 #ffbf0e;

Solution 4:

I know this is old. But yeah. I prefer much shorter solution, than Giona answer

[contenteditable] {
    border-bottom: 1px solid transparent;
    &:focus {outline: none; border-bottom: 1px dashed #000;}
}