Omit values of HTML boolean attributes

There are some HTML boolean attributes such as checked, disabled, readonly, required, selected, etc.
According to the HTML specification, a boolean attribute has three possible declarations. All of them have the same effect:
<input readonly />
<input readonly="" />
<input readonly="readonly" />
true and false are invalid values:
<!-- Not allowed -->
<button disabled="true">...</button>
<button disabled="false">...</button>
The only way to represent a false value is to remove the attribute. Hence, to avoid the incorrect and misleading usages, it's recommended to remove the value:
<input readonly />