Re: [w3c/webcomponents] What's the expected behavior of a class with observedAttributes but no attributeChangedCallback? (#555)

We can answer this by looking at the spec, in particular https://html.spec.whatwg.org/multipage/scripting.html#dom-customelementregistry-define step 10. In this case, the observedAttributes property is ignored (step 10.6 is false, so 10.6.1 doesn't even run). Even if you put a getter, it is not supposed to be triggered, i.e. given

```js
customElements.define("x-x", class extends HTMLElement {
  static get observedAttributes() { console.log("does this happen?"); }
});
```

nothing should be logged to the console (and no error thrown).

Sounds like a bug with the polyfill :)

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