fix: enhance background-position minification and handling for various cases (#1124)

Author: Jungzl

src/lib.rs

@@ -4311,12 +4311,24 @@ mod tests {
     );
     minify_test(
       ".foo { background-position: left 10px center }",
-      ".foo{background-position:10px 50%}",
+      ".foo{background-position:10px}",
     );
     minify_test(
       ".foo { background-position: right 10px center }",
       ".foo{background-position:right 10px center}",
     );
+    minify_test(
+      ".foo { background-position: center top 10px }",
+      ".foo{background-position:50% 10px}",
+    );
+    minify_test(
+      ".foo { background-position: center bottom 10px }",
+      ".foo{background-position:center bottom 10px}",
+    );
+    minify_test(
+      ".foo { background-position: center 10px }",
+      ".foo{background-position:50% 10px}",
+    );
     minify_test(
       ".foo { background-position: right 10px top 20px }",
       ".foo{background-position:right 10px top 20px}",
@@ -4337,6 +4349,26 @@ mod tests {
       ".foo { background-position: bottom right }",
       ".foo{background-position:100% 100%}",
     );
+    minify_test(
+      ".foo { background-position: center top }",
+      ".foo{background-position:top}",
+    );
+    minify_test(
+      ".foo { background-position: center bottom }",
+      ".foo{background-position:bottom}",
+    );
+    minify_test(
+      ".foo { background-position: left center }",
+      ".foo{background-position:0}",
+    );
+    minify_test(
+      ".foo { background-position: right center }",
+      ".foo{background-position:100%}",
+    );
+    minify_test(
+      ".foo { background-position: 20px center }",
+      ".foo{background-position:20px}",
+    );
 
     minify_test(
       ".foo { background: url('img-sprite.png') no-repeat bottom right }",

src/values/position.rs

@@ -203,11 +203,32 @@ impl ToCss for Position {
         // `center` is assumed if omitted.
         x_lp.to_css(dest)
       }
+      (
+        &HorizontalPosition::Side {
+          side: HorizontalPositionKeyword::Left,
+          offset: Some(ref x_lp),
+        },
+        y,
+      ) if y.is_center() => {
+        // `left 10px center` => `10px` (omit Y when center)
+        x_lp.to_css(dest)
+      }
       (&HorizontalPosition::Side { side, offset: None }, y) if y.is_center() => {
         let p: LengthPercentage = side.into();
         p.to_css(dest)
       }
       (x, y_pos @ &VerticalPosition::Side { offset: None, .. }) if x.is_center() => y_pos.to_css(dest),
+      (
+        &HorizontalPosition::Center,
+        y_pos @ &VerticalPosition::Side {
+          side: VerticalPositionKeyword::Bottom,
+          offset: Some(_),
+        },
+      ) => {
+        // `center bottom 10px` must keep the keyword form
+        dest.write_str("center ")?;
+        y_pos.to_css(dest)
+      }
       (&HorizontalPosition::Side { side: x, offset: None }, &VerticalPosition::Side { side: y, offset: None }) => {
         let x: LengthPercentage = x.into();
         let y: LengthPercentage = y.into();