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<!DOCTYPE html>
<html>
<head>
<style>
[class^="top"] {
    background: yellow;
}
</style>
</head>
<body>

<h1 class="top-header">Welcome</h1>
<p class="top-text">Hello world!</p>
<p class="topcontent">Are you learning CSS?</p>

<p><b>Note:</b> For [<i>attribute</i>^=<i>value</i>] to work in IE8 and earlier, a DOCTYPE must be declared.</p>

</body>
</html>