A SPA can track the mouse cursor, as well as stealing form content that was left unsubmitted.
an MPA can use javascript that does exactly the same.
You don’t even need JavaScript. CSS has Hover attribute and you can use it to fire beacons.
Also validation is usually built on both client and server for the same things. Like if you have a password complexity validation. Its both on UX and the server otherwise it will be a very terrible UX experience.