Re: [webcomponents] [Custom Elements] the `extends` property cannot be replaced (#326)

Here is another use case.

I'm using is-attributes in my [SVG editor](https://chrome.google.com/webstore/detail/boxy-svg/gaoogdonmngmdlbinmiclicjpbjhgomg) to implement custom shapes such as a star, a ring or an arrow by extending the `<path>` element. For example when user draws a star, I'm generating the following markup using my custom [star shape API](https://gist.github.com/jarek-foksa/151418a0b4ab6ebab330):

```svg
<path is="bx-starpath" d="..." data-bx-cx="204" data-bx-cy="153" data-bx-r1="41" data-bx-r2="103" data-bx-arms="7"></path>
```
You might argue that I could achieve the same result with <bx-star> custom element that wraps the path:

```svg
<bx-star cx="204" cy="153" r1="41" r2="103" arms="7">
  <-- Shadow DOM -->
  <path d="..."></path>
</bx-star>
```

The problem with the latter approach is that the custom element will render correctly only in environment that supports scripting and includes my implementation of the star shape, it won't work when loaded into another SVG editor such as Adobe Illustrator or when set as a background image.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/326#issuecomment-147342265