[css-forms-1] `appearance: base` is not a "New value"

[cdoublev]

It was added 6 months ago with #10691.

For CSS tools processing New value fields, the result is that appearance is defined with base twice:

none | auto | base | <compat-auto> | <compat-special> | base

Is it planned to add other new values soon? Otherwise could this new value be removed?