diff --git a/e2e/app/index.html b/e2e/app/index.html
index fe1fe10..996a579 100644
--- a/e2e/app/index.html
+++ b/e2e/app/index.html
@@ -103,17 +103,6 @@
       }
     };
 
-    // `Color` serializes untagged: a named colour or hex arrives as a string,
-    // an rgba arrives as `{r, g, b, a}`.
-    function cssColor(value) {
-      if (value == null) return null;
-      if (typeof value === 'string') return value;
-      if (typeof value === 'object' && value.r != null) {
-        return `rgba(${value.r}, ${value.g}, ${value.b}, ${value.a})`;
-      }
-      return String(value);
-    }
-
     function renderWidget(node) {
       if (!node || typeof node !== 'object') {
         return document.createTextNode(String(node || ''));
@@ -576,7 +565,7 @@
           if (node.padding != null) el.style.padding = node.padding + 'px';
           if (node.width) el.style.width = node.width;
           if (node.height) el.style.height = node.height;
-          const background = cssColor(node.background);
+          const background = node.background;
           if (background) {
             el.style.background = background;
             el.setAttribute('data-background', background);
@@ -594,7 +583,7 @@
             el.setAttribute('data-size', String(node.size));
             el.style.fontSize = node.size + 'px';
           }
-          const color = cssColor(node.color);
+          const color = node.color;
           if (color) {
             el.setAttribute('data-color', color);
             el.style.color = color;
diff --git a/rusty/src/shared/color.rs b/rusty/src/shared/color.rs
index 7944e16..4960ee0 100644
--- a/rusty/src/shared/color.rs
+++ b/rusty/src/shared/color.rs
@@ -1,8 +1,13 @@
-use serde::{Deserialize, Serialize};
+use serde::de::{self, Visitor};
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
 /// A color value supporting named colors, hex, and RGBA.
-#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
-#[serde(untagged)]
+///
+/// Serializes to a CSS color string (`"primary"`, `"#ff0000"`, `"rgba(1, 2, 3, 0.5)"`) and
+/// deserializes back from one, so `Serialize` and [`Color::to_css`] agree and the
+/// value round-trips. A `#[serde(untagged)]` derive would emit named/hex as strings
+/// but rgba as an object `{"r":1,"g":2,"b":3,"a":0.5}`, forcing clients to sniff the type.
+#[derive(Debug, Clone, PartialEq)]
 pub enum Color {
     Named(NamedColor),
     Hex(String),
@@ -23,6 +28,39 @@ pub enum NamedColor {
     Black,
 }
 
+impl NamedColor {
+    /// The camelCase wire name, matching the `rename_all` derive.
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            NamedColor::Primary => "primary",
+            NamedColor::Secondary => "secondary",
+            NamedColor::Success => "success",
+            NamedColor::Warning => "warning",
+            NamedColor::Danger => "danger",
+            NamedColor::Info => "info",
+            NamedColor::Muted => "muted",
+            NamedColor::White => "white",
+            NamedColor::Black => "black",
+        }
+    }
+
+    /// Parse a camelCase wire name back into a variant.
+    pub fn parse(value: &str) -> Option<NamedColor> {
+        match value {
+            "primary" => Some(NamedColor::Primary),
+            "secondary" => Some(NamedColor::Secondary),
+            "success" => Some(NamedColor::Success),
+            "warning" => Some(NamedColor::Warning),
+            "danger" => Some(NamedColor::Danger),
+            "info" => Some(NamedColor::Info),
+            "muted" => Some(NamedColor::Muted),
+            "white" => Some(NamedColor::White),
+            "black" => Some(NamedColor::Black),
+            _ => None,
+        }
+    }
+}
+
 impl Color {
     pub fn hex(value: &str) -> Self {
         Color::Hex(value.to_string())
@@ -31,6 +69,71 @@ impl Color {
     pub fn rgba(r: u8, g: u8, b: u8, a: f32) -> Self {
         Color::Rgba { r, g, b, a }
     }
+
+    /// Render as a CSS color. This is the wire form: `Serialize` emits exactly
+    /// this string.
+    pub fn to_css(&self) -> String {
+        match self {
+            Color::Named(named) => named.as_str().to_string(),
+            Color::Hex(hex) => hex.clone(),
+            Color::Rgba { r, g, b, a } => format!("rgba({}, {}, {}, {})", r, g, b, a),
+        }
+    }
+
+    /// Parse a CSS color string produced by [`Color::to_css`].
+    pub fn parse_css(value: &str) -> Option<Color> {
+        let trimmed = value.trim();
+        if let Some(named) = NamedColor::parse(trimmed) {
+            return Some(Color::Named(named));
+        }
+        if trimmed.starts_with('#') {
+            return Some(Color::Hex(trimmed.to_string()));
+        }
+        if let Some(args) = trimmed
+            .strip_prefix("rgba(")
+            .and_then(|rest| rest.strip_suffix(')'))
+        {
+            let parts: Vec<&str> = args.split(',').map(str::trim).collect();
+            if parts.len() == 4 {
+                return Some(Color::Rgba {
+                    r: parts[0].parse().ok()?,
+                    g: parts[1].parse().ok()?,
+                    b: parts[2].parse().ok()?,
+                    a: parts[3].parse().ok()?,
+                });
+            }
+        }
+        None
+    }
+}
+
+impl Serialize for Color {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        serializer.serialize_str(&self.to_css())
+    }
+}
+
+impl<'de> Deserialize<'de> for Color {
+    fn deserialize<D: Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
+        struct ColorVisitor;
+
+        impl Visitor<'_> for ColorVisitor {
+            type Value = Color;
+
+            fn expecting(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+                f.write_str(
+                    "a CSS color such as \"primary\", \"#ff0000\" or \"rgba(1, 2, 3, 0.5)\"",
+                )
+            }
+
+            fn visit_str<E: de::Error>(self, value: &str) -> Result<Color, E> {
+                Color::parse_css(value)
+                    .ok_or_else(|| de::Error::invalid_value(de::Unexpected::Str(value), &self))
+            }
+        }
+
+        deserializer.deserialize_str(ColorVisitor)
+    }
 }
 
 impl From<NamedColor> for Color {
@@ -59,4 +162,52 @@ mod tests {
             panic!("Expected hex color");
         }
     }
+
+    #[test]
+    fn test_color_serialize_matches_to_css() {
+        assert_eq!(
+            serde_json::to_string(&Color::Named(NamedColor::Primary)).unwrap(),
+            "\"primary\""
+        );
+        assert_eq!(
+            serde_json::to_string(&Color::Hex("#ff0000".to_string())).unwrap(),
+            "\"#ff0000\""
+        );
+        assert_eq!(
+            serde_json::to_string(&Color::Rgba {
+                r: 1,
+                g: 2,
+                b: 3,
+                a: 0.5
+            })
+            .unwrap(),
+            "\"rgba(1, 2, 3, 0.5)\""
+        );
+    }
+
+    #[test]
+    fn test_color_round_trips() {
+        let named = Color::Named(NamedColor::Primary);
+        let json = serde_json::to_string(&named).unwrap();
+        assert_eq!(serde_json::from_str::<Color>(&json).unwrap(), named);
+
+        let hex = Color::Hex("#ff0000".to_string());
+        let json = serde_json::to_string(&hex).unwrap();
+        assert_eq!(serde_json::from_str::<Color>(&json).unwrap(), hex);
+
+        let rgba = Color::Rgba {
+            r: 1,
+            g: 2,
+            b: 3,
+            a: 0.5,
+        };
+        let json = serde_json::to_string(&rgba).unwrap();
+        assert_eq!(serde_json::from_str::<Color>(&json).unwrap(), rgba);
+    }
+
+    #[test]
+    fn test_color_deserialize_rejects_object_and_garbage() {
+        assert!(serde_json::from_str::<Color>("{\"r\":1,\"g\":2,\"b\":3,\"a\":1.0}").is_err());
+        assert!(serde_json::from_str::<Color>("\"notacolor\"").is_err());
+    }
 }
diff --git a/rusty/src/shared/types.rs b/rusty/src/shared/types.rs
index 5507157..f1e474c 100644
--- a/rusty/src/shared/types.rs
+++ b/rusty/src/shared/types.rs
@@ -1,12 +1,17 @@
-use serde::{Deserialize, Serialize};
+use serde::de::{self, Visitor};
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
 /// Unique identifier for views and widgets.
 pub type ViewId = uuid::Uuid;
 pub type WidgetId = uuid::Uuid;
 
 /// Size specification for widgets.
-#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
-#[serde(untagged)]
+///
+/// Serializes to a CSS length string (`"8px"`, `"50%"`, `"auto"`) and
+/// deserializes back from one, so `Serialize` and [`Size::to_css`] agree and the
+/// value round-trips. A `#[serde(untagged)]` derive would collapse `Px(8.0)` and
+/// `Percent(8.0)` to a bare `8.0` and `Auto` to `null`.
+#[derive(Debug, Clone, PartialEq)]
 pub enum Size {
     Px(f64),
     Percent(f64),
@@ -14,10 +19,8 @@ pub enum Size {
 }
 
 impl Size {
-    /// Render as a CSS length. Widgets serialize sizes through this rather than
-    /// through `Serialize`: the derive is `untagged`, so `Px(8.0)` and
-    /// `Percent(8.0)` both emit a bare `8.0` and `Auto` emits `null`, leaving a
-    /// client unable to tell pixels from percent or `Auto` from unset.
+    /// Render as a CSS length. This is the wire form: `Serialize` emits exactly
+    /// this string.
     pub fn to_css(&self) -> String {
         match self {
             Size::Px(px) => format!("{}px", px),
@@ -25,16 +28,53 @@ impl Size {
             Size::Auto => "auto".to_string(),
         }
     }
+
+    /// Parse a CSS length string produced by [`Size::to_css`].
+    pub fn parse_css(value: &str) -> Option<Size> {
+        let trimmed = value.trim();
+        if trimmed.eq_ignore_ascii_case("auto") {
+            return Some(Size::Auto);
+        }
+        if let Some(num) = trimmed.strip_suffix('%') {
+            return num.trim().parse::<f64>().ok().map(Size::Percent);
+        }
+        if let Some(num) = trimmed.strip_suffix("px") {
+            return num.trim().parse::<f64>().ok().map(Size::Px);
+        }
+        None
+    }
+}
+
+impl Serialize for Size {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        serializer.serialize_str(&self.to_css())
+    }
+}
+
+impl<'de> Deserialize<'de> for Size {
+    fn deserialize<D: Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
+        struct SizeVisitor;
+
+        impl Visitor<'_> for SizeVisitor {
+            type Value = Size;
+
+            fn expecting(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+                f.write_str("a CSS length such as \"8px\", \"50%\" or \"auto\"")
+            }
+
+            fn visit_str<E: de::Error>(self, value: &str) -> Result<Size, E> {
+                Size::parse_css(value)
+                    .ok_or_else(|| de::Error::invalid_value(de::Unexpected::Str(value), &self))
+            }
+        }
+
+        deserializer.deserialize_str(SizeVisitor)
+    }
 }
 
 /// Render an optional [`Size`] as its CSS string, for `#[prop(with = ...)]`.
 ///
 /// `Size`'s derived `Serialize` is `untagged` and therefore lossy on the wire
-/// (`Px(8.0)` and `Percent(8.0)` both become `8.0`), so widgets emit the CSS
-/// form instead. See [`Size::to_css`].
-pub fn size_css(size: &Option<Size>) -> Option<String> {
-    size.as_ref().map(Size::to_css)
-}
 
 /// Density level for widget rendering.
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Serialize, Deserialize)]
@@ -80,10 +120,40 @@ mod tests {
     }
 
     #[test]
-    fn test_size_serialize_is_lossy_so_widgets_use_to_css() {
-        // Documents why `to_css` exists: `untagged` collapses the variants.
-        assert_eq!(serde_json::to_string(&Size::Px(8.0)).unwrap(), "8.0");
-        assert_eq!(serde_json::to_string(&Size::Percent(8.0)).unwrap(), "8.0");
-        assert_eq!(serde_json::to_string(&Size::Auto).unwrap(), "null");
+    fn test_size_serialize_matches_to_css() {
+        assert_eq!(serde_json::to_string(&Size::Px(8.0)).unwrap(), "\"8px\"");
+        assert_eq!(
+            serde_json::to_string(&Size::Percent(8.0)).unwrap(),
+            "\"8%\""
+        );
+        assert_eq!(serde_json::to_string(&Size::Auto).unwrap(), "\"auto\"");
+    }
+
+    #[test]
+    fn test_size_round_trips() {
+        let px = Size::Px(8.0);
+        let json = serde_json::to_string(&px).unwrap();
+        assert_eq!(serde_json::from_str::<Size>(&json).unwrap(), px);
+
+        let percent = Size::Percent(50.0);
+        let json = serde_json::to_string(&percent).unwrap();
+        assert_eq!(serde_json::from_str::<Size>(&json).unwrap(), percent);
+
+        let auto = Size::Auto;
+        let json = serde_json::to_string(&auto).unwrap();
+        assert_eq!(serde_json::from_str::<Size>(&json).unwrap(), auto);
+    }
+
+    #[test]
+    fn test_size_deserialize_rejects_bare_number_and_unknown_unit() {
+        assert!(serde_json::from_str::<Size>("8.0").is_err());
+        assert!(serde_json::from_str::<Size>("null").is_err());
+        assert!(serde_json::from_str::<Size>("\"8em\"").is_err());
+    }
+
+    #[test]
+    fn test_size_option_none_is_null() {
+        let none: Option<Size> = None;
+        assert_eq!(serde_json::to_string(&none).unwrap(), "null");
     }
 }
diff --git a/rusty/src/widgets/container.rs b/rusty/src/widgets/container.rs
index fc0bb39..b69d1b0 100644
--- a/rusty/src/widgets/container.rs
+++ b/rusty/src/widgets/container.rs
@@ -88,8 +88,8 @@ impl WidgetData for Container {
                 .map(|c| serde_json::to_value(c).unwrap_or_default())
                 .collect::<Vec<_>>(),
             "padding": self.padding,
-            "width": self.width.as_ref().map(Size::to_css),
-            "height": self.height.as_ref().map(Size::to_css),
+            "width": self.width,
+            "height": self.height,
             "background": self.background,
             "border": self.border,
             "rounded": self.rounded,
diff --git a/rusty/src/widgets/image.rs b/rusty/src/widgets/image.rs
index 402fc32..e69b286 100644
--- a/rusty/src/widgets/image.rs
+++ b/rusty/src/widgets/image.rs
@@ -59,8 +59,8 @@ impl WidgetData for Image {
             "id": self.id,
             "src": self.src,
             "alt": self.alt,
-            "width": self.width.as_ref().map(Size::to_css),
-            "height": self.height.as_ref().map(Size::to_css),
+            "width": self.width,
+            "height": self.height,
         })
     }
 
diff --git a/rusty/src/widgets/layout.rs b/rusty/src/widgets/layout.rs
index 4f18519..ab5b66d 100644
--- a/rusty/src/widgets/layout.rs
+++ b/rusty/src/widgets/layout.rs
@@ -36,10 +36,10 @@ pub struct Layout {
     #[prop]
     #[serde(skip_serializing_if = "Option::is_none")]
     pub columns: Option<usize>,
-    #[prop(with = "crate::shared::size_css")]
+    #[prop]
     #[serde(skip_serializing_if = "Option::is_none")]
     pub width: Option<Size>,
-    #[prop(with = "crate::shared::size_css")]
+    #[prop]
     #[serde(skip_serializing_if = "Option::is_none")]
     pub height: Option<Size>,
     #[prop]
diff --git a/rusty/src/widgets/skeleton.rs b/rusty/src/widgets/skeleton.rs
index 5ca1997..bf4f4c2 100644
--- a/rusty/src/widgets/skeleton.rs
+++ b/rusty/src/widgets/skeleton.rs
@@ -8,10 +8,10 @@ use serde::{Deserialize, Serialize};
 pub struct Skeleton {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub id: Option<String>,
-    #[prop(with = "crate::shared::size_css")]
+    #[prop]
     #[serde(skip_serializing_if = "Option::is_none")]
     pub width: Option<Size>,
-    #[prop(with = "crate::shared::size_css")]
+    #[prop]
     #[serde(skip_serializing_if = "Option::is_none")]
     pub height: Option<Size>,
 }