opacity

значения свойства:

1
number
inherit

css/style.css

* {
margin: 0;
padding: 0;
}
.main {
width: 600px;
height: 500px;
position: absolute;
left: 50%;
top: 50%;
margin-left: -300px;
margin-top: -250px;
}
.a {
width: 298px;
height: 98px;
font-size: 26px;
background-color: #00F;
border: 1px solid #808080;
text-align: center;
line-height: 98px;
float: left;
}
.b {
width: 298px;
height: 98px;
font-size: 26px;
border: 1px solid #808080;
text-align: center;
line-height: 98px;
float: left;
}
.c {
width: 600px;
}
#div1 {
opacity: 1;
}
#div4 {
opacity: 0.3;
}
#div5 {
opacity: 0.0;
}

* {
 margin: 0;
 padding: 0;
}

.main {
 width: 600px;
 height: 500px;
 position: absolute;
 left: 50%;
 top: 50%;
 margin-left: -300px;
 margin-top: -250px;
}

.a {
 width: 298px;
 height: 98px;
 font-size: 26px;
 background-color: #00F;
 border: 1px solid #808080;
 text-align: center;
 line-height: 98px;
 float: left;
}

.b {
 width: 298px;
 height: 98px;
 font-size: 26px;
 border: 1px solid #808080;
 text-align: center;
 line-height: 98px;
 float: left;
}

.c {
 width: 600px;
}

#div1 {
 opacity: 1;
}

#div4 {
 opacity: 0.3;
}

#div5 {
 opacity: 0.0;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="main">
<div class="c">
<div id="div1" class="a">Hello World!</div>
<div class="b">opacity = 1</div>
</div>
<div class="c">
<div id="div2" class="a">Hello World!</div>
<div class="b">opacity = 0.7</div>
</div>
<div class="c">
<div id="div3" class="a">Hello World!</div>
<div class="b">opacity = 0.5</div>
</div>
<div class="c">
<div id="div4" class="a">Hello World!</div>
<div class="b">opacity = 0.3</div>
</div>
<div class="c">
<div id="div5" class="a">Hello World!</div>
<div class="b">opacity = 0</div>
</div>
</div>
<script>
/*JavaScript*/
document.getElementById("div2").style.opacity = "0.7";
/*jQuery*/
$(document).ready(function () {
$("#div3").css({ "opacity": "0.5" });
});
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="main">
<div class="c">
<div id="div1" class="a">Hello World!</div>
<div class="b">opacity = 1</div>
</div>
<div class="c">
<div id="div2" class="a">Hello World!</div>
<div class="b">opacity = 0.7</div>
</div>
<div class="c">
<div id="div3" class="a">Hello World!</div>
<div class="b">opacity = 0.5</div>
</div>
<div class="c">
<div id="div4" class="a">Hello World!</div>
<div class="b">opacity = 0.3</div>
</div>
<div class="c">
<div id="div5" class="a">Hello World!</div>
<div class="b">opacity = 0</div>
</div>
</div>
<script>
/*JavaScript*/
document.getElementById("div2").style.opacity = "0.7";
/*jQuery*/
$(document).ready(function () {
$("#div3").css({ "opacity": "0.5" });
});
</script>
</body>
</html>