Can you elaborate more in this? Years ago, I used to primarily do Rails development. Recently I built some web apps that use a JVM backend (one app uses Java & Spring and the other Kotlin & Micronaut) and a React frontend. One thing I ended up really missing issue the the frameworks, especially with disjointed fronted, don't solve the standard issue of a request sending an invalid form entry and showing the validation errors on the form. I ended up building my own implementation of that which of course also requires a convention on message format. Since most apps need to solve this it's so weird to be that frameworks nowadays don't solve this out of the box.