empty element tag 空元件标签
EndElement represents the element's closing tag (or simply the end of the element's markup, if it is an empty element).
EndElement表示元素的结束标记(或元素标记的结束，如果是空元素的话)。
Remember, the DOM will create well-formed HTML, meaning that the element is currently empty, with both a starting and ending tag.
要记住，DOM会创建结构良好的HTML，就是说这个目前为空的元素包括起始和结束标签。
Moreover, an element that has no content is represented by an empty child list, but a self-closed tag is represented by None. Listing 2 shows this structure in action.
而一个没有内容的元素由一个空的子列表表示，但是一个自封闭的标记由None表示。
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