style: Serialize a 0/0 ratio as 0/0 in all value stages.

Based on the update of github.com/w3c/csswg-drafts/issues/5084,
a 0/0 ratio will serialize as 0/0 in all value stages.

Differential Revision: https://phabricator.services.mozilla.com/D93182

components/style/gecko/media_features.rs

@@ -87,8 +87,12 @@ fn eval_aspect_ratio_for<F>(
 where
     F: FnOnce(&Device) -> Size2D<Au>,
 {
+    // A ratio of 0/0 behaves as the ratio 1/0, so we need to call used_value()
+    // to convert it if necessary.
+    // FIXME: we may need to update here once
+    // https://github.com/w3c/csswg-drafts/issues/4954 got resolved.
     let query_value = match query_value {
-        Some(v) => v,
+        Some(v) => v.used_value(),
         None => return true,
     };
 

components/style/values/computed/position.rs

@@ -15,7 +15,7 @@ use crate::values::generics::position::PositionOrAuto as GenericPositionOrAuto;
 use crate::values::generics::position::Ratio as GenericRatio;
 use crate::values::generics::position::ZIndex as GenericZIndex;
 pub use crate::values::specified::position::{GridAutoFlow, GridTemplateAreas, MasonryAutoFlow};
-use crate::Zero;
+use crate::{One, Zero};
 use std::cmp::{Ordering, PartialOrd};
 use std::fmt::{self, Write};
 use style_traits::{CssWriter, ToCss};
@@ -89,6 +89,16 @@ impl Ratio {
     pub fn new(a: f32, b: f32) -> Self {
         GenericRatio(a.into(), b.into())
     }
+
+    /// Returns the used value. A ratio of 0/0 behaves as the ratio 1/0.
+    /// https://drafts.csswg.org/css-values-4/#ratios
+    pub fn used_value(self) -> Self {
+        if self.0.is_zero() && self.1.is_zero() {
+            Ratio::new(One::one(), Zero::zero())
+        } else {
+            self
+        }
+    }
 }
 
 /// A computed value for the `aspect-ratio` property.

components/style/values/specified/position.rs

@@ -951,13 +951,6 @@ impl Parse for Ratio {
             _ => One::one(),
         };
 
-        // The computed value of a <ratio> is the pair of numbers provided, unless
-        // both numbers are zero, in which case the computed value is the pair (1, 0)
-        // (same as 1 / 0).
-        // https://drafts.csswg.org/css-values-4/#ratios
-        if a.is_zero() && b.is_zero() {
-            return Ok(GenericRatio(One::one(), Zero::zero()));
-        }
-        return Ok(GenericRatio(a, b));
+        Ok(GenericRatio(a, b))
     }
 }