Boolean attributes
You'll sometimes see attributes written without values — this is perfectly
allowed. These are called boolean attributes, and they can only have one value,
which is generally the same as the attribute name. As an example, take the
disabled attribute, which you can assign to form input elements if you want
them to be disabled (greyed out) so the user can't enter any data in them.
As a shorthand, it is perfectly allowable to write this as follows (we've also included a non-disabled form input element for reference to give you more of an idea of what is going on):
<input type="text" value="I am disabled" disabled="disabled" /> <input type="text" value="I am also disabled" disabled> <input type="text" value="I am enabled">