Forms used to be checked field by field, with the browser's own bubbles doing part of the work and each page filling the gaps. All of that is now a single module that every form uses: the contact forms, the sign in dialog, anything added later.

The rules are declared on the field itself (required, an email address, a minimum length, two fields that must match, an accepted file type, a maximum file size, a number of files), and every message comes from the same translations as the rest of the page.

The browser popups are gone on purpose. The error is drawn under the field that caused it, the first broken field gets the focus, and a submission with a mistake never reaches the server. A refusal that comes back from the server is shown on the same field, so there is only one place to look.