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html5 では属性(attribute)が真偽値の場合には属性値の省略が出来る。
値に true, false を指定することは出来ないので、値を false にするときは属性を書かない。

以下はすべて同じ。

<label><input type=checkbox checked name=cheese disabled> Cheese</label>
<label><input type=checkbox checked=checked name=cheese disabled=disabled> Cheese</label>
<label><input type='checkbox' checked name=cheese disabled=""> Cheese</label>

2.5 Common microsyntaxes — HTML5より引用:

The values “true” and “false” are not allowed on
boolean attributes. To represent a false value, the attribute has to
be omitted altogether.

Here is an example of a checkbox that is checked and disabled.
The checked and disabled attributes are the
boolean attributes.

<label><input type=checkbox checked name=cheese disabled> Cheese</label>

This could be equivalently written as this:

<label><input type=checkbox checked=checked name=cheese disabled=disabled> Cheese</label>

You can also mix styles; the following is still equivalent:

<label><input type='checkbox' checked name=cheese disabled=""> Cheese</label>