has="mixin" Idea for managing mixins without conflicting with existing classes or is= attribute. A followup to #662 #509 re is=.

Goal: address practical need for mixins with existing and custom elements, hook into element lifecycle, not conflict with various libraries, expectations in the wild.

...Sorter...
customElements.mixin('sortable', Sorter);


<any has="sortable"></any>
<any has="sortable filterable whatever"></any>