Represents an element with the att attribute whose value is a white space-separated list of words, one of which is exactly "val".

当元素的“ att ”属性值是一个以空格分割的“字”的列表，并且其中之一为“ val ”，匹配成功。

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Match when the element sets the "att" attribute, whatever the value of the attribute.

当元素设置了"att"属性时匹配成功，而不论属性值是什么。

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