CodePen Embed - Groups of selector

<h1>header 1</h1>
<h2>header 2</h2>
<h3>header 3</h3>

h1, h2, h3 { color: red; }

/* 上面那句等同於下面三句 */
/* h1 { color: red; }
h2 { color: red; }
h3 { color: red; } */

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