Styling form control pickers

[mozilla-apprentice]

Resolutions were made for csswg-drafts/#10440.

Styling form control pickers

• RESOLVED: `appearance:base` will apply to the in-page parts (only) of all form controls (including `<select>`), and, to avoid forwards-compat problems, must ship simultaneously on all form controls.
• RESOLVED: `::picker(keyword) { appearance: base }` will apply to form control pickers as an opt-in to styleable pickers of the named control type, with the control-type keyword allowing per-control-type incremental shipping of styleable pickers. To avoid forwards-compat problems, we will consider a keywordless variant only once all form controls can be opted in.
• RESOLVED: If appearance: base on all in-page controls is not ready in time for <select>, we may add appearance: base-select whose behavior is equivalent to appearance: base, but only applies to <select> and its picker (::picker(select)), individually.

Discussion.

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