Propose UA requirements for Shadow DOM mapping

[rahimabdi]

Subissue from this closed issue: #3.

The element expresses a broad range of controls specified by its type attribute that may present additional user-agent supplied content/controls, e.g.,:

• :

For each of these and complex s generally, there should be updated mappings for all user-agent generated shadow DOM content.

Additional notes:

• The bug @scottaohara observes in this related issue still exists for Chromium: Review/revise mappings for input type=file #421.
• Labelling UA-generated shadow DOM elements may be a challenge as the user agent-generated name may be immutable.