How to apply border around shape created by child divs

Question

I have a number of overlapping divs which I am treating as a single shape. The shape needs to be semi-opaque, so I have housed them in a parent div and applied opacity to the parent div like so:

#top_housing_bg_div {
  opacity: 0.55;
  filter: alpha(opacity=55);
}

#top_bg_div {
  z-index: 9999;
  padding: 3px;
  position: absolute;
  width: -webkit-calc(100% - 14px - 9px);
  width: -moz-calc(100% - 14px - 9px);
  width: calc(100% - 14px - 9px);
  top: 7px;
  left: 7px;
  background-color: black;
  color: white;
  font-size: 12px;
  border-radius: 5px;
  overflow: hidden;
}

#logo_bg_div {
  z-index: 9999;
  position: absolute;
  top: 12px;
  left: 12px;
  background-color: black;
  border-radius: 5px;
  overflow: hidden;
  width: 50px;
  height: 50px;
}

<div id="top_housing_bg_div">
  <div id="top_bg_div">
    &nbsp;
  </div>
  <div id="logo_bg_div">
    <!--empty for now-->
  </div>
</div>

I need to have a single border that follows the outline of that semi-opaque shape (as opposed to each sub-div having its own border which encroaches into the semi-opaque body of the other elements) and I have tried - and failed - to do so by using box-shadow on the parent div, setting border etc.

Is it possible to achieve what I'm trying to achieve?

Answer 1

As you already started adding specific values ( height, width, position etc. ) , you can use a pseudo-element to achieve this goal by adding it on top of the smaller div, and so, covering the border of the div.

See below ( hope i understood correctly what you wanted )

body {
margin:0
}

#logo_bg_div:before {
  height: 20px;
  top: -1px;
  width: calc(100% + 2px);
  background: black;
  position: absolute;
  left: -1px;
  content: "";
}


#top_housing_bg_div {
  opacity: 0.55;
  filter: alpha(opacity=55);
}

#top_bg_div {
  z-index: 9999;
  padding: 3px;
  position: absolute;
  width: -webkit-calc(100% - 14px - 9px);
  width: -moz-calc(100% - 14px - 9px);
  width: calc(100% - 14px - 9px);
  top: 7px;
  left: 7px;
  background-color: black;
  color: white;
  font-size: 12px;
  border-radius: 5px;
  overflow: hidden;
  border: 1px solid red;
}

#logo_bg_div {
  z-index: 9999;
  position: absolute;
  top: 12px;
  left: 12px;
  background-color: black;
  border-radius: 5px;
  width: 50px;
  height: 50px;
  border: 1px solid red;
  position: relative
}

<div id="top_housing_bg_div">
  <div id="top_bg_div">
    &nbsp;
  </div>
  <div id="logo_bg_div">
    <!--empty for now-->
  </div>
</div>