[foul]

That demonstration as per OP is dumb or targeted to React-ists. You can, with HTMX, do the classic AJAX submit with offline validation.

In the last years, for every layer of web development, what I saw was that a big smelly pile of problems with bad websites and webapps, be it MPA or SPA, was not a matter of bad developers on the product, but more a problem of bad, sometimes plain evil, developers on systems sold to developers to build their product upon. Boilerplate for apps, themes, ready-made app templates are largely garbage, bloat, and prone to supply chain attacks of any sort.