<h1> The Basic Language of the Web: HTML</h1>
<h2> The Basic Language of the Web: HTML</h2>
<h3> The Basic Language of the Web: HTML</h3>
<h4> The Basic Language of the Web: HTML</h4>
<h5> The Basic Language of the Web: HTML</h5>
<h6> The Basic Language of the Web: HTML</h6>效果：
规范
- 每个网页应该只有一个 h1 标签