By doing what XML-with-schema did 15 years ago: give compile-time type checking, making sure at least the syntax and types are valid.

Ever had CloudFormation yell at you because some function like "Sub!" only accepts alphanumeric characters? Created complicated constructs because there are no variables, and very limited conditionals? Copy-pasted templates like an insane person because there is no way to abstract anything?

Not my idea of fun. All of this is solved by using a proper programming language.