这个选择符用于选择出特定的单一元素；id在一个文档中应该是惟一的。

This selector is used to single out a particular element; IDs should be unique within a document.

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换句话来说，它们不是同一元素，就算它们看起来像是，并且拥有相同的标识符也不行。

In other words, these are not the same element even though they look the same and have identical characteristics.

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例如，如下的属性选择符匹配那些指定了“ title ”属性的H1元素，而不论其值为什么？

For example, the following attribute selector matches all H1 elements that specify the "title" attribute, whatever its value.

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