Comment by danieldk
I don't think forms are a bridge too far, it was very common that forms were provided as PDF and it is more convenient for the sender and receiver to fill the fields on a computer for readability, etc. before printing.
However, forms could be handled by a very simple DSL that would be easy to write a safe interpreter for.
JavaScript is already a simple language. There's no requirement to have a JIT even. What makes you believe a custom language would be any safer or better in another way?