Add intrinsic size computation for inline formatting contexts

components/style/values/computed/length.rs

@@ -170,6 +170,12 @@ impl LengthPercentage {
         self.length
     }
 
+    /// Returns the percentage component of this `calc()`
+    #[inline]
+    pub fn percentage_component(&self) -> Percentage {
+        Percentage(self.clamping_mode.clamp(self.percentage.0))
+    }
+
     /// Return the percentage value as CSSFloat.
     #[inline]
     pub fn percentage(&self) -> CSSFloat {

components/style/values/computed/percentage.rs

@@ -64,6 +64,12 @@ impl Zero for Percentage {
     }
 }
 
+impl std::ops::AddAssign for Percentage {
+    fn add_assign(&mut self, other: Self) {
+        self.0 += other.0
+    }
+}
+
 impl ToCss for Percentage {
     fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
     where