A number of attributes are boolean attributes 1).
The values true and false are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.
Example with the defer attribute
<script defer/>
<script/>
If you use a XML library and that you try to enforce xhtml, you may get an error saying that a value is mandatory
Setting the value to true does not follow the HTML specification but XML one
<!-- not html compliant, only xhtml/xml compliant -->
<script defer="true"/>
You need to set the doctype accordingly
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>