make it responsive

[Garbee]

Looking at the element it creates an embed with a hard-coded size. By allowing custom attributes to be used to give a size (and handle their changing events) we can allow developers to get the size they need seamlessly.

While carrying this out, we'll follow the best practices for custom elements where applicable.

[paulirish]

Yah this seems required. Web.dev used a variant on my approach which handled dimensions better than me.

[paulirish]

web.dev uses this padding-top trick (which is written explicitly for the youtube case) https://blog.theodo.com/2018/01/responsive-iframes-css-trick/

although i like chris's adaptation in https://css-tricks.com/responsive-iframes/