Builds on attach-shadow, adopt-styles and ShadowTemplate, this commit adds ShadowHost and finally CustomElement.
CustomElement is a renderless component to help with the creation of native HTML Custom Elements along with runtime type checking and self-documentation for attributes, slots, cssprops and cssparts. As you will probably see there is a little more work to come here. But in the same breath, everything would be fine to go in as is.
John Cowen