tag + class

<header>页眉</header>

<main>主体</main>

<footer>页脚</footer>

<tag class="header">页眉</tag>
<tag class="main">主体</tag>
<tag class="footer">页脚</tag>

<div class="header">页眉</div>
<div class="main">主体</div>
<div class="footer">页脚</div>