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。

Remember, the DOM will create well-formed HTML, meaning that the element is currently empty, with both a starting and ending tag.

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将空的元素转化成起始-结束标签对。

Empty elements are converted to start-end tag pairs.

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将空的元素转化成起始-结束标签对。

Empty elements are converted to start-end tag pairs.

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