"placeholder_node": {
  "type": "object",
  "additionalProperties": false,
  "required": [
    "type",
    "attrs"
  ],
  "properties": {
    "type": {
      "enum": ["placeholder"]
    },
    "attrs": {
      "type": "object",
      "additionalProperties": false,
      "required": ["text"],
      "properties": {}
    }
  }
}

 "type": {
  "enum": ["placeholder"]
}

 "attrs": {
  "type": "object",
  "additionalProperties": false,
  "required": ["text"],
  "properties": {
    "text": {"type": "string"}
  }
}

 "text": {"type": "string"}