diff --git a/components/style/gecko/conversions.rs b/components/style/gecko/conversions.rs
index 89cce99beb0..345e199a9a6 100644
--- a/components/style/gecko/conversions.rs
+++ b/components/style/gecko/conversions.rs
@@ -22,8 +22,8 @@ use crate::values::computed::transform::Matrix3D;
 use crate::values::computed::url::ComputedImageUrl;
 use crate::values::computed::{Angle, Gradient, Image};
 use crate::values::computed::{Integer, LengthPercentage};
+use crate::values::computed::{Length, Percentage, TextAlign};
 use crate::values::computed::{LengthPercentageOrAuto, NonNegativeLengthPercentageOrAuto};
-use crate::values::computed::{Percentage, TextAlign};
 use crate::values::generics::box_::VerticalAlign;
 use crate::values::generics::grid::{TrackListValue, TrackSize};
 use crate::values::generics::image::{CompatMode, GradientItem, Image as GenericImage};
@@ -35,6 +35,11 @@ use style_traits::values::specified::AllowedNumericType;
 
 impl From<LengthPercentage> for nsStyleCoord_CalcValue {
     fn from(other: LengthPercentage) -> nsStyleCoord_CalcValue {
+        debug_assert!(
+            other.was_calc ||
+                other.percentage.is_none() ||
+                other.unclamped_length() == Length::zero()
+        );
         let has_percentage = other.percentage.is_some();
         nsStyleCoord_CalcValue {
             mLength: other.unclamped_length().to_i32_au(),
diff --git a/components/style/values/specified/position.rs b/components/style/values/specified/position.rs
index d13bc579f02..b9006ee799b 100644
--- a/components/style/values/specified/position.rs
+++ b/components/style/values/specified/position.rs
@@ -263,11 +263,12 @@ impl<S: Side> ToComputedValue for PositionComponent<S> {
                 let length = length.to_computed_value(context);
                 let p = Percentage(1. - length.percentage());
                 let l = -length.unclamped_length();
+                // We represent `<end-side> <length>` as `calc(100% - <length>)`.
                 ComputedLengthPercentage::with_clamping_mode(
                     l,
                     Some(p),
                     length.clamping_mode,
-                    length.was_calc,
+                    /* was_calc = */ true,
                 )
             },
             PositionComponent::Side(_, Some(ref length)) |