feat(parse): Support legacy pseudo-elements

src/parse.spec.ts

@@ -61,4 +61,10 @@ describe("Parse", () => {
 
         expect(() => parse("/*/")).toThrowError("Comment was not terminated");
     });
+
+    it("should support legacy pseudo-elements with single colon", () => {
+        expect(parse(":before")).toEqual([
+            [{ name: "before", data: null, type: "pseudo-element" }],
+        ]);
+    });
 });

src/parse.ts

@@ -67,6 +67,19 @@ const unpackPseudos = new Set([
     "host-context",
 ]);
 
+/**
+ * Pseudo elements defined in CSS Level 1 and CSS Level 2 can be written with
+ * a single colon; eg. :before will turn into ::before.
+ *
+ * @see {@link https://www.w3.org/TR/2018/WD-selectors-4-20181121/#pseudo-element-syntax}
+ */
+const pseudosToPseudoElements = new Set([
+    "before",
+    "after",
+    "first-line",
+    "first-letter",
+]);
+
 /**
  * Checks whether a specific selector is a traversal.
  * This is useful eg. in swapping the order of elements that
@@ -472,10 +485,20 @@ function parseSelector(
                                 ? readValueWithParenthesis()
                                 : null,
                     });
-                    continue;
+                    break;
                 }
 
                 const name = getName(1).toLowerCase();
+
+                if (pseudosToPseudoElements.has(name)) {
+                    tokens.push({
+                        type: SelectorType.PseudoElement,
+                        name,
+                        data: null,
+                    });
+                    break;
+                }
+
                 let data: DataType = null;
 
                 if (