是强制性的
XHTML 文档必须进行 XHTML 文档类型声明（XHTML DOCTYPE declaration）。
您可以在菜鸟教程的标签参考手册中找到完整的 XHTML 文档类型。
, , , 和 <body> 元素也必须存在，并且必须使用 <html> 中的 xmlns 属性为文档规定 xml 命名空间。</div><div>下面的例子展示了带有最少的必需标签的 XHTML 文档：</div><div style="background-color: #F9F9F9;"><span style="color: olive;"><</span><span style="color: gray;">!</span><span style="color: #00008B;">DOCTYPE</span><span style="color: gray;"> </span><span style="color: #00008B;">html</span><span style="color: gray;"> </span><span style="color: #00008B;">PUBLIC</span><span style="color: gray;"> </span><span style="color: #8B0000;">"</span><span style="color: #AA1111;">-//W3C//DTD XHTML 1.0 Transitional//EN</span><span style="color: #8B0000;">"</span><span style="color: gray;"></span><span style="color: #8B0000;">"</span><span style="color: #AA1111;"><a href="http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" target="_blank">http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd</a></span><span style="color: #8B0000;">"</span><span style="color: olive;">></span><span style="color: gray;"> </span><span style="color: olive;"><</span><span style="color: green;">html</span><span style="color: gray;"> </span><span style="color: #00008B;">xmlns</span><span style="color: gray;">=</span><span style="color: #8B0000;">"</span><span style="color: #AA1111;"><a href="http://www.w3.org/1999/xhtml" target="_blank">http://www.w3.org/1999/xhtml</a></span><span style="color: #8B0000;">"</span><span style="color: olive;">></span><span style="color: gray;"> </span><span style="color: olive;"><</span><span style="color: green;">head</span><span style="color: olive;">></span><span style="color: gray;"> </span><span style="color: olive;"><</span><span style="color: green;">meta</span><span style="color: gray;"> </span><span style="color: #00008B;">charset</span><span style="color: gray;">=</span><span style="color: #8B0000;">"</span><span style="color: #AA1111;">utf-8</span><span style="color: #8B0000;">"</span><span style="color: olive;">></span><span style="color: gray;"> </span><span style="color: olive;"><</span><span style="color: green;">title</span><span style="color: olive;">></span><span style="color: gray;">文档标题</span><span style="color: olive;"></</span><span style="color: green;">title</span><span style="color: olive;">></span><span style="color: gray;"></span><span style="color: olive;"></</span><span style="color: green;">head</span><span style="color: olive;">></span><span style="color: gray;"> </span><span style="color: olive;"><</span><span style="color: green;">body</span><span style="color: olive;">></span><span style="color: gray;">文档内容</span><span style="color: olive;"></</span><span style="color: green;">body</span><span style="color: olive;">></span><span style="color: gray;"> </span><span style="color: olive;"></</span><span style="color: green;">html</span><span style="color: olive;">></span></div>
