diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index 905816f0b77..423a79be5dc 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -1263,6 +1263,9 @@ impl ToCss for PropertyDeclaration {
             PropertyDeclaration::WithVariables(_, ref with_variables) => {
                 // https://drafts.csswg.org/css-variables/#variables-in-shorthands
                 match with_variables.from_shorthand {
+                    // Normally, we shouldn't be printing variables here if they came from
+                    // shorthands. But we should allow properties that came from shorthand
+                    // aliases. That also matches with the Gecko behavior.
                     Some(shorthand) if shorthand.flags().contains(SHORTHAND_ALIAS_PROPERTY) =>
                         dest.write_str(&*with_variables.css)?,
                     None => dest.write_str(&*with_variables.css)?,
@@ -1345,6 +1348,10 @@ impl PropertyDeclaration {
                         Some(&*with_variables.css)
                     } else { None }
                 } else {
+                    // Normally, longhand property that doesn't come from a shorthand
+                    // should return None here. But we return Some to longhands if they
+                    // came from a shorthand alias. Because for example, we should be able to
+                    // get -moz-transform's value from transform.
                     if shorthand.flags().contains(SHORTHAND_ALIAS_PROPERTY) {
                         return Some(&*with_variables.css);
                     }