If a boolean attribute is present, it is always true. Boolean attributes include autofocus, inert, checked, disabled, required, reversed, allowfullscreen, default, loop, autoplay, controls, muted, readonly, multiple, and selected
Boolean values can either be omitted, set to an empty string, or be the name of the attribute; but the value doesn’t have to actually be set to the string true. All values, including true, false, and , while invalid, will resolve to true.
These three tags are equivalent
<input required>
<input required="">
<input required="required">