Question

My understanding is that width: 100% lets the element’s width be the same as its parent’s, whereas width: inherit does that only when the parent’s width is explicitly specified. Is this understanding correct?

If so, it looks to me that when width: inherit works, then width: 100% would always work, so you could always use the latter. Then, what is the purpose of writing width: inherit? When does it become useful?

If my understanding is wrong, what is the difference between the two?

Similarly with height.

Answer 1

See jsfiddle http://jsfiddle.net/bt5nj/2/ and http://jsfiddle.net/bt5nj/3/

width:inherit inherits width that defined by parent.

HTML:

  <div id="parent">
      <div id="child"></div>
  </div>​

CSS:

  #parent {
      width:50%;
      height:30px;
  }

  #child {
      width:inherit;
      height:100%;
      background-color:red;
  }

This makes child width 25%, but if I redefine it with width:100% it will define width of child 50%.

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