feat: support border-spacing (#294)

Author: yisibl

src/lib.rs

@@ -257,6 +257,73 @@ mod tests {
     };
   );
 
+  #[test]
+  pub fn test_border_spacing() {
+    minify_test(
+      r#"
+      .foo {
+        border-spacing: 0px;
+      }
+    "#,
+      indoc! {".foo{border-spacing:0}"
+      },
+    );
+    minify_test(
+      r#"
+      .foo {
+        border-spacing: 0px 0px;
+      }
+    "#,
+      indoc! {".foo{border-spacing:0}"
+      },
+    );
+
+    minify_test(
+      r#"
+      .foo {
+        border-spacing: 12px   0px;
+      }
+    "#,
+      indoc! {".foo{border-spacing:12px 0}"
+      },
+    );
+
+    minify_test(
+      r#"
+      .foo {
+        border-spacing: calc(3px * 2) calc(5px * 0);
+      }
+    "#,
+      indoc! {".foo{border-spacing:6px 0}"
+      },
+    );
+
+    minify_test(
+      r#"
+      .foo {
+        border-spacing: calc(3px * 2) max(0px, 8px);
+      }
+    "#,
+      indoc! {".foo{border-spacing:6px 8px}"
+      },
+    );
+
+    // TODO: The `<length>` in border-spacing cannot have a negative value, 
+    // we may need to implement NonNegativeLength like Servo does.
+    // Servo Code: https://github.com/servo/servo/blob/08bc2d53579c9ab85415d4363888881b91df073b/components/style/values/specified/length.rs#L875
+    // CSSWG issue: https://lists.w3.org/Archives/Public/www-style/2008Sep/0161.html
+    // `border-spacing = <length> <length>?`
+    minify_test(
+      r#"
+      .foo {
+        border-spacing: -20px;
+      }
+    "#,
+      indoc! {".foo{border-spacing:-20px}"
+      },
+    );
+  }
+
   #[test]
   pub fn test_border() {
     test(

src/properties/mod.rs

@@ -805,6 +805,8 @@ define_properties! {
   "inset-inline": InsetInline(InsetInline) shorthand: true,
   "inset": Inset(Inset) shorthand: true,
 
+  "border-spacing": BorderSpacing(Size2D<Length>),
+
   "border-top-color": BorderTopColor(CssColor) [logical_group: BorderColor, category: Physical],
   "border-bottom-color": BorderBottomColor(CssColor) [logical_group: BorderColor, category: Physical],
   "border-left-color": BorderLeftColor(CssColor) [logical_group: BorderColor, category: Physical],