How to make div's percentage width relative to parent div and not viewport

Answer 1

Specifying a non-static position, e.g., position: absolute/relative on a node means that it will be used as the reference for absolutely positioned elements within it http://jsfiddle.net/E5eEk/1/

See https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layout/Positioning#Positioning_contexts

We can change the positioning context — which element the absolutely positioned element is positioned relative to. This is done by setting positioning on one of the element's ancestors.

#outer {
  min-width: 2000px; 
  min-height: 1000px; 
  background: #3e3e3e; 
  position:relative
}

#inner {
  left: 1%; 
  top: 45px; 
  width: 50%; 
  height: auto; 
  position: absolute; 
  z-index: 1;
}

#inner-inner {
  background: #efffef;
  position: absolute; 
  height: 400px; 
  right: 0px; 
  left: 0px;
}

<div id="outer">
  <div id="inner">
    <div id="inner-inner"></div>
  </div>
</div>

Answer 2

Use position: relative on the parent element.

Also note that had you not added any position attributes to any of the divs you wouldn't have seen this behavior. Juan explains further.