How to make a child div transparent?

Question

Suppose there is a div, say "parent-div". The parent div has a background color. What if the child div, "child-div", needs to be set with a transparent background,such that it is set with the background image of the grandparent div, with class name "wrapper"?

I know that a child div can inherit css properties from parent div, but how do I set the background to transparent, making the whole picture appear like the parent-div has a hole in it?

.wrapper{
  background-image: url('http://media.istockphoto.com/photos/medium-golden-brown-wood-texture-background-picture-id513694258?k=6&m=513694258&s=170667a&w=0&h=xETakP7VjpAtRj9e6rJRYNqw_AJLZ9ovLlC4ebR5BOQ=');
}

.parent-div{
  width: 100px;
  height: 100px;
  background: #ff0000;
  padding: 10px;
  margin: auto;
}

.child-div{
  width: 60%;
  height: 60%;
  margin: auto;
  background: transparent;
  border: 1px solid;
}

<div class="wrapper">
  <div class="parent-div">
    <div class="child-div">
    </div>
  </div>
</div>

Answer 1

Don't apply background on .parent-div.

Instead use a large value of box-shadow on .child-div and add overflow: hidden on .parent-div to hide unwanted shadow effect.

Following css will do the work:

.parent-div {
  overflow: hidden;
}
.child-div {
  box-shadow: 0 0 0 500px #f00;
}

.wrapper {
  background-image: url('http://media.istockphoto.com/photos/medium-golden-brown-wood-texture-background-picture-id513694258?k=6&m=513694258&s=170667a&w=0&h=xETakP7VjpAtRj9e6rJRYNqw_AJLZ9ovLlC4ebR5BOQ=');
}

.parent-div {
  overflow: hidden;
  width: 100px;
  height: 100px;
  padding: 10px;
  margin: auto;
}

.child-div {
  box-shadow: 0 0 0 500px #f00;
  border: 1px solid;
  width: 60%;
  height: 60%;
  margin: auto;
  
}

<div class="wrapper">
  <div class="parent-div">
    <div class="child-div">
    </div>
  </div>
</div>