That one is a dud. Most others work for me, though.
explicit border:2px solid var(--border); on the input which takes priority over the class :
.demo-uv-input:user-invalid { border-color: var(--red-muted); background: rgba(239,68,68,.04); }
YMMV but when corrected it works for me on latest Edge, Chrome, Firefox (Windows versions)
But it's the jagged edges, the unorthodox and surprising prickly bits, that tear open a hole in the inattention of your reader, that actually gets your ideas into their heads.
He lacks (or lost thru disuse) technical expertise on the subject, so he uses more and more fuzzy words, leaky analogies, buzzwords.
This maybe why AI generated content has so much success among leaders and politicians.