JSON Schema

{
  "type": "object",
  "title": "Comment",
  "properties": {
    "name":  {
      "title": "Name",
      "type": "string"
    },
    "email":  {
      "title": "Email",
      "type": "string",
      "pattern": "^\\S+@\\S+$",
      "description": "Email will be used for evil."
    },
    "comment": {
      "title": "Comment",
      "type": "string",
      "maxLength": 20,
      "validationMessage": "Don't be greedy!"
    }
  },
  "required": ["name","email","comment"]
}
        

Form definition

[
  "name",
  "email",
  {
    "key": "comment",
    "type": "textarea",
    "placeholder": "Make a comment"
  },
  {
    "type": "submit",
    "title": "OK"
  }
]