style: Make appearance: button behave like auto on various elements.

Differential Revision: https://phabricator.services.mozilla.com/D83434

components/style/values/specified/box.rs

@@ -1842,6 +1842,31 @@ pub enum Appearance {
     Count,
 }
 
+/// The effect of `appearance: button` on an element.
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    Hash,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToCss,
+    ToComputedValue,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(u8)]
+pub enum ButtonAppearance {
+    /// `appearance: button` means the element is rendered with button
+    /// appearance.
+    Allow,
+    /// `appearance: button` is treated like `appearance: auto`.
+    Disallow,
+}
+
 /// A kind of break between two boxes.
 ///
 /// https://drafts.csswg.org/css-break/#break-between