1. Normal form validation (data, type-email/url/integer/datetime, required/optional)
2. Custom validation (Dependencies on value of other elements. Ex: if country = 'India', render few more fields etc..
2. Decision tree (realtime render/re-render sections, injecting/firing rules depend on the data)
3. Responsive, drag and drop, form meta data can be stored as JSON and can be used to create the form on the fly.
4. Supporting of file browse, link, images, applying different styles for different forms..
The Rule Engine should not tight to UI/HTML DOM. Independent representation for any business.
Preference libraries can be Angular JS or similar data binding.
Found something in git : https://github.com/rsamec/business-rules-engine
Another company site you can refer (but it is exactly not like this) http://www.doforms.com/