Dim entire screen except a fixed area?

I want to create a tutorial which will lead the user exactly where to click. I'm trying to cover the entire screen with a <div> which will dim all elements except a specific region which is in a fixed width, height, top and left.

The problem is, I cannot find a way to "cancel" the parent's background-color (which is also transparent).

In the snipped below, hole is the div that is supposed to be without any background-color, including its parent's.

Can this be accomplished at all? Any ideas?

#bg{
   background-color:gray;
   opacity:0.6;
   width:100%;
   height:100vh;
}
#hole{
   position:fixed;
   top:100px;
   left:100px;
   width:100px;
   height:100px;
}

<div id="bg">
    <div id="hole"></div>
</div>

Here's a mockup image of what I'm trying to achieve:

You could do it with just one div and give it a box-shadow.

EDIT: as @Nick Shvelidze pointed out, you should consider adding pointer-events: none

Added vmax value for box-shadow as @Prinzhorn suggested

div {
  position: fixed;
  width: 200px;
  height: 200px;
  top: 50px;
  left: 50px;
  /* for IE */
  box-shadow: 0 0 0 1000px rgba(0,0,0,.3);
  /* for real browsers */
  box-shadow: 0 0 0 100vmax rgba(0,0,0,.3);
  pointer-events: none;
}

<div></div>

You can create an SVG which is filled with a path that has the hole where you need it to. But I guess than you need to find a way to handle clicks, since all of them will be targeted to the overlaid svg. I thing document.getElementFromPoint can help you here. mdn

This can also be done similarly to @VilleKoo's answer, but with a border.

div {
    border-style: solid;
    border-color: rgba(0,0,0,.3);
    pointer-events: none;
    position: absolute;
    top: 0; bottom: 0; left: 0; right: 0;
    border-width: 40px 300px 50px 60px;
}

<div></div>