boolean attributes

In XHTML, all attributes have to have values, even if those values are redundant.

For example, you’d often see markup like this:
<input type="text" disabled="disabled" />

In HTML5, attributes that are either "on" or "off" (called Boolean attributes) can simply be specified with no value. So, the above input element could now be written as follows:
<input type="text" disabled>
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