How to apply an opacity without affecting a child element with html/css?

Question

You can use opacity in combination with background color, like this:

#container {
    border: solid gold 1px;   
    width: 400px;
    height: 200px;
    background:rgba(56,255,255,0.1);
}

#box {
    border: solid silver 1px;
    margin: 10px;
    width: 300px;
    height: 100px;
    background:rgba(205,206,255,0.1);
}

<div id="container">
    containter text
    <div id="box">
        box text
    </div>
</div>

​Live demo

As far as I know you can't do it in a simple way. There a couple of options here:

1. Use absolute positioning to position box "inside" the container.

   #container {
       opacity: 0.3;
       background-color: #777788;
       position: absolute;
       top: 100px;
       left: 100px;
       height: 150px;
       width: 300px;
   }
   #box {
       opacity: 1;
       background-color: #ffffff;
       position: absolute;
       top: 110px;
       left: 110px;
       height: 130px;
       width: 270px;
   }

   <div id="container"></div>
   <div id="box">
       <p>Something in here</p>
   </div>

2. Use Javascript - almost the same as above, but position and size don't have to be hardcoded.

Try using rgba as a 'pre content' overlay to your image, its a good way to keep things responsive and for none of the other elements to be effected.

header #inner_header_post_thumb {
  background-position: center;
  background-size: cover;
  position: relative;
  background-image: url(https://images.pexels.com/photos/730480/pexels-photo-730480.jpeg?w=1260&h=750&auto=compress&cs=tinysrgb);
  border-bottom: 4px solid #222;
}

header #inner_header_post_thumb .dark_overlay {
  position: relative;
  left: 0;
  top: 0;
  right: 0;
  bottom: 0;
  background-color: rgba(0, 0, 0, 0.75);
}

header #inner_header_post_thumb .dark_overlay .container .header-txt {
  padding-top: 220px;
  padding-bottom: 220px;
  color: #ffffff;
  text-align:center;
}

header #inner_header_post_thumb .dark_overlay .container .header-txt h1 {
  font-size: 40px;
  color: #ffffff;
}

header #inner_header_post_thumb .dark_overlay .container .header-txt h3 {
  font-size: 24px;
  color: #ffffff;
  font-weight: 300;
}

header #inner_header_post_thumb .dark_overlay .container .header-txt p {
  font-size: 18px;
  font-weight: 300;
}

header #inner_header_post_thumb .dark_overlay .container .header-txt p strong {
  font-weight: 700;
}

<header>
  <div id="inner_header_post_thumb">
    <div class="dark_overlay">
      <div class="container">
        <div class="row header-txt">
          <div class="col-xs-12 col-sm-12">
            <h1>Title On Dark A Underlay</h1>
            <h3>Have a dark background image overlay without affecting other elements</h3>
            <p>No longer any need to re-save backgrounds as .png ... <strong>Awesome</strong></p>

          </div>
        </div>
      </div>
    </div>
  </div>
</header>

See a working codepen here

Using background-color: rgba(#777788, 0.3); instead of opacity could maybe fix the problem.