Prevent form validation mixin from generating invalid CSS (#28447)
Prevent form validation mixin from generating invalid CSS (#28447)

Extrapolates the previously used selector into a new mixin that only generates valid CSS by creating psuedo-classes for `valid` and `invalid`, and nothing else.

Fixes #28380.
kvlsrg/bootstrap,tagliala/bootstrap,stanwmusic/bootstrap,coliff/bootstrap,twbs/bootstrap,GerHobbelt/bootstrap,kvlsrg/bootstrap,inway/bootstrap,zalog/bootstrap,tagliala/bootstrap,kvlsrg/bootstrap,tjkohli/bootstrap,coliff/bootstrap,m5o/bootstrap,GerHobbelt/bootstrap,peterblazejewicz/bootstrap,tjkohli/bootstrap,twbs/bootstrap,zalog/bootstrap,GerHobbelt/bootstrap,stanwmusic/bootstrap,peterblazejewicz/bootstrap,inway/bootstrap,stanwmusic/bootstrap,inway/bootstrap,tagliala/bootstrap,peterblazejewicz/bootstrap,zalog/bootstrap,m5o/bootstrap,stanwmusic/bootstrap