style: Implement CSS round() function

Co-Authored-By: Emilio Cobos Álvarez <emilio@crisal.io>

Differential Revision: https://phabricator.services.mozilla.com/D156742

components/style/values/computed/length.rs

@@ -17,7 +17,7 @@ use crate::values::{specified, CSSFloat};
 use crate::Zero;
 use app_units::Au;
 use std::fmt::{self, Write};
-use std::ops::{Add, AddAssign, Div, Mul, MulAssign, Neg, Sub, SubAssign};
+use std::ops::{Add, AddAssign, Div, Mul, MulAssign, Neg, Rem, Sub, SubAssign};
 use style_traits::{CSSPixel, CssWriter, ToCss};
 
 pub use super::image::Image;
@@ -402,6 +402,15 @@ impl Neg for CSSPixelLength {
     }
 }
 
+impl Rem for CSSPixelLength {
+    type Output = Self;
+
+    #[inline]
+    fn rem(self, other: Self) -> Self {
+        CSSPixelLength::new(self.0 % other.0)
+    }
+}
+
 impl Sub for CSSPixelLength {
     type Output = Self;
 

components/style/values/computed/length_percentage.rs

@@ -624,10 +624,10 @@ impl PartialOrd for CalcLengthPercentageLeaf {
 }
 
 impl calc::CalcNodeLeaf for CalcLengthPercentageLeaf {
-    fn is_negative(&self) -> bool {
+    fn unitless_value(&self) -> f32 {
         match *self {
-            Self::Length(ref l) => l.px() < 0.,
-            Self::Percentage(ref p) => p.0 < 0.,
+            Self::Length(ref l) => l.px(),
+            Self::Percentage(ref p) => p.0,
         }
     }
 
@@ -657,6 +657,28 @@ impl calc::CalcNodeLeaf for CalcLengthPercentageLeaf {
         Ok(())
     }
 
+    fn try_op<O>(&self, other: &Self, op: O) -> Result<Self, ()>
+    where
+        O: Fn(f32, f32) -> f32,
+    {
+        match (self, other) {
+            (
+                &CalcLengthPercentageLeaf::Length(ref one),
+                &CalcLengthPercentageLeaf::Length(ref other),
+            ) => Ok(CalcLengthPercentageLeaf::Length(Length::new(op(
+                one.px(),
+                other.px(),
+            )))),
+            (
+                &CalcLengthPercentageLeaf::Percentage(one),
+                &CalcLengthPercentageLeaf::Percentage(other),
+            ) => Ok(CalcLengthPercentageLeaf::Percentage(Percentage(op(
+                one.0, other.0,
+            )))),
+            _ => Err(()),
+        }
+    }
+
     fn mul_by(&mut self, scalar: f32) {
         match *self {
             Self::Length(ref mut l) => *l = *l * scalar,

components/style/values/computed/percentage.rs

@@ -77,6 +77,30 @@ impl std::ops::AddAssign for Percentage {
     }
 }
 
+impl std::ops::Add for Percentage {
+    type Output = Self;
+
+    fn add(self, other: Self) -> Self {
+        Percentage(self.0 + other.0)
+    }
+}
+
+impl std::ops::Sub for Percentage {
+    type Output = Self;
+
+    fn sub(self, other: Self) -> Self {
+        Percentage(self.0 - other.0)
+    }
+}
+
+impl std::ops::Rem for Percentage {
+    type Output = Self;
+
+    fn rem(self, other: Self) -> Self {
+        Percentage(self.0 % other.0)
+    }
+}
+
 impl ToCss for Percentage {
     fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
     where