[whatwg/dom] Attributes not ready on element construction? (#522)

I've got a Custom Element that is used in HTML markup with existing attribute, f.e.

```html
<some-element has="...">
</some-element>
```

and in the class definition I have this:

```js
    constructor( options = {} ) {
        super(options)
        console.log(' --- has attribute (sync)?', this.getAttribute('has')) // undefined

        Promise.resolve().then(() => {
            console.log(' --- has attribute (microtask)?', this.getAttribute('has')) // undefined
        })

        setTimeout(() => {
            console.log(' --- has attribute (macrotask)?', this.getAttribute('has')) // works!
        }, 0)
    }
```

As you can see, I am not able to see existing attribute during construction, which seems really strange because the element already has attributes in the HTML markup, so I expected it to have attributes after calling `super()`.

Furthermore, I find it super strange that I have to defer to a _macro_ task rather than a _micro_ task to detect initial attributes.

Is this as expected?

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