为什么它可以设定高度，但又不像 <h1> 这些元素，那种感觉就像它是个“半内联的（内联：inline-level）元素。HTML 4是这样描述的：
The following elements may also be considered block-level elements since they may contain block-level elements:
DD – Definition description
DT – Definition term
FRAMESET – Frameset
LI – List item
TBODY
