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operator	is implied
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Summary

Boolean True when the attribute or data attribute has no value set, and false otherwise. Replacing is with isnt will reverse the results.

Syntax

  attribute attribute-name element-qualifier*
     ((item | @) numeric-expression)? (is | isnt) implied

An attribute test that succeeds when:

• the reference element attribute has no value set, either in the start tag or in the element declaration
• the data attribute has no value set, either in the start tag or in the element declaration