Initial draft of Layers in Cascade 5

Hi all,

I finished a first draft of adding Cascade Layers to Cascading and Inheritance Level 5, and would be glad for any review:

The draft spec: https://drafts.csswg.org/css-cascade-5/

I updated:

• The Cascade Sorting Order: https://drafts.csswg.org/css-cascade-5/#cascade-sort
• Layer details: https://drafts.csswg.org/css-cascade-5/#layering

Related issues (most are also linked inline from the spec):

https://github.com/w3c/csswg-drafts/issues?q=is%3Aopen+label%3Acss-cascade-5+layers

Cheers,
-Miriam