[adrr]

I don't understand how spa is different than vanilla web app in terms of user analytics? A beacon is a beacon. Whether its img tag with a 1x1 transparent gif or an ajax call.

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.

[ragall]

A SPA can track the mouse cursor, as well as stealing form content that was left unsubmitted.

[detaro]

an MPA can use javascript that does exactly the same.

[adrr]

You don’t even need JavaScript. CSS has Hover attribute and you can use it to fire beacons.

[adrr]

Htmlx has a type ahead attribute that you can add to form elements and send the contents to an endpoint without the user explicit submitting the form.