UI pseudo-classes - Learn web development
the :user-invalid pseudo-class, when supported, will be similar to :invalid, but with better user experience.
... if the value is valid when the input receives focus, the element may match :invalid as the user enters data if the value is temporarily invalid, but will only match :user-invalid when the element loses focus.
... if the value was originally invalid, it will match both :invalid and :user-invalid for the whole duration of the focus.
... in a similar manner to :invalid, it will stop matching :user-invalid if the value does become valid.

:-moz-ui-invalid - CSS: Cascading Style Sheets
the specification for :user-invalid is still in progress.
... specifications specification status comment selectors level 4the definition of ':user-invalid' in that specification.
... working draft :user-invalid ...

Pseudo-classes - CSS: Cascading Style Sheets
erminate :in-range :invalid :is() :lang() :last-child :last-of-type :left :link :local-link :not() :nth-child() :nth-col() :nth-last-child() :nth-last-col() :nth-last-of-type() :nth-of-type() :only-child :only-of-type :optional :out-of-range :past :placeholder-shown :read-only :read-write :required :right :root :scope :state() :target :target-within :user-invalid :valid :visited :where() specifications specification status comment fullscreen api living standard defined :fullscreen.
... selectors level 4 working draft defined :any-link, :blank, :local-link, :scope, :drop, :current, :past, :future, :placeholder-shown, :user-invalid, :nth-col(), :nth-last-col(), :is() and :where().

<input>: The Input (Form Input) element - HTML: Hypertext Markup Language
:user-invalid similar to :invalid, but is activated on blur.