<h1>Tiêu đề 1</h1>
<h2>Tiêu đề 2</h2>
<h3>Tiêu đề 3</h3>
h1{
background-color: red;
opacity: 0.5;
}
h2{
background-color: red;
opacity: 0.7;
}
h3{
background-color: red;
opacity: 1;
}
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