<input>: The Input (Form Input) element - HTML: Hypertext Markup Language
required validitystate.valuemissing occurs when the required attribute is present but the value is null or radio or checkbox is not checked.
...the validity object includes: validitystate.valuemissing validitystate.typemismatch validitystate.patternmismatch validitystate.toolong validitystate.tooshort validitystate.rangeunderflow validitystate.rangeoverflow validitystate.stepmismatch validitystate.badinput validitystate.valid validitystate.customerror for each of these boolean properties, a value of true indicates that the specified reason validation may have failed is true, with t...

<input type="checkbox"> - HTML: Hypertext Markup Language
if the checkbox has the required attribute, but is not checked, then validitystate.valuemissing will be true.