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 />

See also