CSS - margin top of 50% is more than 50%

Question

I want a child div with a op-margin via percentage. The y-position shall be 50% percent of the parent div. But it is somehow more. Why isn't it 50% ?

js fiddle

HTML

<div class="content">
  <header></header>
</div>

CSS

.content {
  width: 300px;
  height: 200px;
  background: blue;
  position: relative;
  clear: both;
 }

.content header {
   width: 100px;
   height: 100px;
   margin-top: 50%;
   background: red;
   position: relative;
   float: left;
}

Answer 1

This is because when it comes to top/bottom margins and paddings in percentages, the values will be taken as the fractional width of the parent element, not the height. According to the W3C definition:

The [margin] percentage is calculated with respect to the width of the generated box's containing block. Note that this is true for 'margin-top' and 'margin-bottom' as well. If the containing block's width depends on this element, then the resulting layout is undefined in CSS 2.1.

This question has been addressed before in StackOverflow - see here.

---

Suggestion: If you would want to position an element in the center vertically, you can use the following trick instead:

1. Position the child element absolutely
2. Declare dimensions for the parent, such that the parent's dimensions do not rely on those of the child
3. Position the child element 50% from the top
4. Use a nifty trick of CSS 2D translation.

Here's the modified CSS from your fiddle that works:

.content header {
    width: 100px;
    height: 100px;
    top: 50%;
    background: red;
    position: absolute;
    -webkit-transform: translate(0, -50%);
    transform: translate(0, -50%);
}

http://jsfiddle.net/teddyrised/73xkT/7/