Auto merge of #15702 - shubDhond:master, r=upsuper

Invalid three value positions are no longer accepted

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- Position parse no longer accepts invalid three value positions

---
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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 #15488 .

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- [X] There are tests for these changes OR

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components/style/values/specified/position.rs

@@ -187,13 +187,11 @@ impl Parse for Position {
             } else {
                 // Handle 3 value background position there are several options:
                 if let PositionCategory::LengthOrPercentage = category(&first) {
-                    // "length keyword length"
-                    Position::new(Some(first), Some(third), None, Some(second))
+                    Err(())
                 } else {
                     if let PositionCategory::LengthOrPercentage = category(&second) {
                         if let PositionCategory::LengthOrPercentage = category(&third) {
-                            // "keyword length length"
-                            Position::new(Some(second), Some(third), Some(first), None)
+                            Err(())
                         } else {
                             // "keyword length keyword"
                             Position::new(Some(second), None, Some(first), Some(third))