在 XHTML 文档中哪些元素是强制性的？

<span style、"font-family: Calibri; font-size: small;">doctype</span><span style="font-family: 宋体; font-size: small;">、</span><span style="font-family: Calibri; font-size: small;">html </span><span style="font-family: 宋体; font-size: small;">以及</span><span style="font-family: Calibri; font-size: small;"> body</span>

<span style、"font-family: Calibri; font-size: small;">doctype</span><span style="font-family: 宋体; font-size: small;">、</span><span style="font-family: Calibri; font-size: small;">html</span><span style="font-family: 宋体; font-size: small;">、</span><span style="font-family: Calibri; font-size: small;">head </span><span style="font-family: 宋体; font-size: small;">以及</span><span style="font-family: Calibri; font-size: small;"> body</span>

<span style、"font-family: Calibri; font-size: small;">doctype</span><span style="font-family: 宋体; font-size: small;">、</span><span style="font-family: Calibri; font-size: small;">html</span><span style="font-family: 宋体; font-size: small;">、</span><span style="font-family: Calibri; font-size: small;">head</span><span style="font-family: 宋体; font-size: small;">、</span><span style="font-family: Calibri; font-size: small;">body </span><span style="font-family: 宋体; font-size: small;">以及</span><span style="font-family: Calibri; font-size: small;"> title</span>