attributes.json

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "name": {
      "type": "string"
    }
  },
  "required": ["name"]
}

component.js

class HelloWorld extends HTMLElement {
  constructor() {
    super();
    this.textContent = `Hello ${this.getAttribute("name")}!`;
  }
}

manifest.json

{
  "name": "hello-world",
  "isDefined": false,
}