<header>
    <h1>ここがヘッダー</h1>
</header>

 <main>
    <h2>ここはメイン</h2>
 </main>
header ::before{
    content: "";
    position: absolute;
    width: 1em;
    height: 1em;
    background-color: pink;
}

h2 ::before{
    content: "";
    position: absolute;
    width: 1em;
    height: 1em;
    background-color: pink;
}

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