style: Hide some appearance values from content.

They're not used internally either, so remove all ability to address them.

I haven't removed the implementation yet, as some of them are quite complex, and
I don't have a mac / windows build. We should do that when this hits release
though.

Differential Revision: https://phabricator.services.mozilla.com/D32488

components/style/values/specified/box.rs

@@ -1129,11 +1129,13 @@ pub enum Appearance {
     ButtonArrowUp,
     /// A rectangular button that contains complex content
     /// like images (e.g. HTML <button> elements)
+    #[css(skip)]
     ButtonBevel,
     /// The focus outline box inside of a button.
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     ButtonFocus,
     /// The caret of a text area
+    #[css(skip)]
     Caret,
     /// A dual toolbar button (e.g., a Back button with a dropdown)
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
@@ -1146,6 +1148,7 @@ pub enum Appearance {
     /// List boxes.
     Listbox,
     /// A listbox item.
+    #[css(skip)]
     Listitem,
     /// Menu Bar background
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
@@ -1165,8 +1168,10 @@ pub enum Appearance {
     /// The dropdown button(s) that open up a dropdown list.
     MenulistButton,
     /// The text part of a dropdown list, to left of button.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     MenulistText,
     /// An editable textfield with a dropdown list (a combobox).
+    #[css(skip)]
     MenulistTextfield,
     /// Menu Popup background.
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]