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

So the goal is to have a 10+ year old client to be able to display _something_ when given a document authored with recent technology in mind?

This requirement doesnt make it any easier for authors to produce documents. Isnt one of the main arguments of custom elements that a document author can expect a certain behavior of a `<google-map>` without understanding the internals? In this case the fallback for old clients, js-disabled clients and pre-upgrade clients is quite easy, just put an `<img>` inside the map and you are mostly fine. for a single `<google-map>` that works fine, but if you go all out on webcomponents, providing fallback content for everything you are basically in the "duplicated layout" case? ofc you can do something like:
`<my-app><strong>Please use a latest browser</strong></my-app>`, but then you are excluding old clients anyway.

Another question is "how good is that _something_ that is displayed?". 
Lets use another example: Within in a form a user is asked to select a medium sized amount of items from a very large set of options. Ideally you want to be able to apply filters and searches on the options so you dont have to scroll into oblivion. you want a quick view of what you actually selected. maybe you want to sort your selection. maybe the options are structured beyond one level of `<optgroup>`. developing a custom element for advanced selection widget would feel super natural from a developers view and also from a documents author view. but good luck to the document author finding a viable "progressive upgrade" path except hiding the element for legacy clients.


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