CSS Background Opacity

Children inherit opacity. It'd be weird and inconvenient if they didn't.

You can use a translucent PNG file for your background image, or use an RGBa (a for alpha) color for your background color.

Example, 50% faded black background:

<div style="background-color:rgba(0, 0, 0, 0.5);">
   <div>
      Text added.
   </div>
</div>

You can use pseudo-elements ::before or ::after to get a semi-transparent background and you can do this with just one container. Use something like this:

<article>
  Text.
</article>

Then apply some CSS:

article {
  position: relative;
  z-index: 1;
}

article::before {
  content: "";
  position: absolute;
  top: 0; 
  left: 0;
  width: 100%; 
  height: 100%;  
  opacity: .4; 
  z-index: -1;
  background: url(path/to/your/image);
}

Example:

body {
  background: red;
}

article {
  position: relative;
  z-index: 1;
}

article:before {
  content: " ";
  position: absolute;
  top: 0; 
  left: 0;
  width: 100%; 
  height: 100px;  
  opacity: .4; 
  z-index: -1;
  background: url(https://31.media.tumblr.com/8ec07e49f33088c2e32c158ca4262eb2/tumblr_n5wav6Tz4R1st5lhmo1_1280.jpg);
}

<article>
  Text.
</article>

Note: You might need to adjust the z-index values.