The easiest way to set transparent background color to a HTML element, without affecting the text-content is to use the rgba() function in CSS.
- The rgba() function is supported by all major browsers (IE 9+).
Example:
<style type="text/css">
#dv1 {
  position: relative;
  width: 78%;
  height: 8em;
  background: rgb(128, 139, 210);     /* For browsers that don't support rgba */
  background-color: rgba(128, 139, 210, 0.5);   /* Transparent background color */
}
</style>

<div id="dv1">
Semi-transparent background color, not affecting text content.<br/>
CSS code snippets from: <a href="http://coursesweb.net/" title="CSS Course">http://coursesweb.net/css/</a>
</div>
Result:
Semi-transparent background color, not affecting text content.
CSS code snippets from: http://coursesweb.net/css/

Transparent Background Image

To create a transparent background image with CSS, the trick is to insert a pseudo element with a image for background and regular opacity, the exact size of the element behind it.
- The pseudo elements are supported by all major browsers (IE 8+).
Example:
<style type="text/css">
#dv2 {
  position: relative;
  width: 78%;
  height: 8em;
  background-color: #fefe90;
  font-size: 1.3em;
}
#dv2::before {
  position: absolute;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  margin: 0;
  content: "";
  background: url('image.jpg');
  background-repeat: no-repeat;
  background-position: 30% 50%;
  opacity: 0.5;
}

/* Settingas for mouse over the element */
#dv2:hover::before {
  position: absolute;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  margin: 0;
  content: "";
  background: url('image.jpg');
  background-repeat: no-repeat;
  background-position: 30% 50%;
  opacity: 0.7;
}
</style>

<div id="dv1">
Semi-transparent background color.<br/>
CSS code snippets from: <a href="http://coursesweb.net/" title="CSS Course">http://coursesweb.net/css/</a>
</div>
Result:
Semi-transparent background image.
CSS code snippets from: http://coursesweb.net/css/

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
Which tag is used to include external CSS file in web page?
<body> <script> <link>
<link href="/templ/style.css" rel="stylesheet" type="text/css" />
Which CSS property sets the text size?
font-weight text-decoration font-size
h2 {
  font-size: 1em;
}
Indicate the JavaScript property that can add HTML code into an element.
text value innerHTML
document.getElementById("someID").innerHTML = "HTML content";
Click on the function that returns the number of characters of a string in PHP.
count() strlen() stristr()
$str = "http://CoursesWeb.net/";
$nr_chr = strlen($str);
echo $nr_chr;       // 22
Transparent Background, but not the text-content on it

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