Auto merge of #16174 - Adynatos:15200-serialization-with-comma, r=Wafflespeanut

Added missing comma in {background,mask}-position

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Fixed #15200 - missing comma in serialized {background,mask}-position with multiple values.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15200 (github issue number if applicable).

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/style/properties/shorthand/background.mako.rs

@@ -234,6 +234,9 @@
                 self.background_position_x.0[i].to_css(dest)?;
                 dest.write_str(" ")?;
                 self.background_position_y.0[i].to_css(dest)?;
+                if i < len - 1 {
+                    dest.write_str(", ")?;
+                }
             }
             Ok(())
         }

components/style/properties/shorthand/mask.mako.rs

@@ -226,7 +226,11 @@
 
             for i in 0..len {
                 self.mask_position_x.0[i].to_css(dest)?;
+                dest.write_str(" ")?;
                 self.mask_position_y.0[i].to_css(dest)?;
+                if i < len - 1 {
+                    dest.write_str(", ")?;
+                }
             }
 
             Ok(())