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<html>
<head>
<title>this is coding</title>
<center>
<h1 align="center"><span style="background-color:#00FF00">this is a nice piece of art
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<style>
.dabutton:focus {
background-color:yellow;
}
</style>
<button class="dabutton"></button> // <-- usage
EASY
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$('#kaufen').click(function() {
$(this).toggleClass('bg-crimson');
});
//css
.bg-crimson {
background-color: white;
color: #092348 !important;
}