Re: [w3c/webcomponents] The is="" attribute is confusing? Maybe we should encourage only ES6 class-based extension. (#509)

Yes, the old style HTC/XBL do not need `is` attribute. Here is the problem list of the old style:

1. No declarative information in markups, so it's not easy to recognize CE, not only agents like search engines but also developers.
1. Attach behavior via stylesheets is rejected by many members of CSS WG, at least in year 1999 (I don't know what the current CSS experts thought.) IMO, It's not clear how one element can have many behaviors attached, and how cascade mechanism works for `behavior: add(xxx)`. 
1. Selectors can be very complex, someone may write `customElements.define('.component-scope .parent-component.foo.bar > .my-button:not(:disabled)', MyButton)` which is terrible and harm to performance. Of coz we can limit it, just like `select` attribute on `content` element in ShadowDOM V0, but I had to say such special rule is confusing.


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