1.不允许写的结束符的标签: area/basebr/col/command/embed/hr/img/input/link/meta/param/source/track/wbr 2.可以省略结束符的标签 li/dt/dd/p/rt/optgruop/option/colgrup/thread/tb/td/th 3.可以完全省略的标签 html/head/body/colgroup/tbody
