Make use of generics for transform-origin

This allows us to preserve keywords during serialisation.

components/style/values/generics/mod.rs

@@ -19,6 +19,7 @@ pub mod grid;
 pub mod image;
 pub mod position;
 pub mod rect;
+pub mod transform;
 
 // https://drafts.csswg.org/css-counter-styles/#typedef-symbols-type
 define_css_keyword_enum! { SymbolsType:

components/style/values/generics/transform.rs

@@ -0,0 +1,45 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Generic types for CSS values that are related to transformations.
+
+use std::fmt;
+use style_traits::ToCss;
+
+/// A generic transform origin.
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
+pub struct TransformOrigin<H, V, Depth> {
+    /// The horizontal origin.
+    pub horizontal: H,
+    /// The vertical origin.
+    pub vertical: V,
+    /// The depth.
+    pub depth: Depth,
+}
+
+impl<H, V, D> TransformOrigin<H, V, D> {
+    /// Returns a new transform origin.
+    pub fn new(horizontal: H, vertical: V, depth: D) -> Self {
+        Self {
+            horizontal: horizontal,
+            vertical: vertical,
+            depth: depth,
+        }
+    }
+}
+
+impl<H, V, D> ToCss for TransformOrigin<H, V, D>
+    where H: ToCss, V: ToCss, D: ToCss,
+{
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write,
+    {
+        self.horizontal.to_css(dest)?;
+        dest.write_str(" ")?;
+        self.vertical.to_css(dest)?;
+        dest.write_str(" ")?;
+        self.depth.to_css(dest)
+    }
+}