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+<!doctype html>
+<title>An+B Parsing</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+
+foo { color: blue; }
+
+</style>
+
+<meta name="author" title="Tab Atkins-Bittner">
+<link rel=help href="https://drafts.csswg.org/css-syntax/#the-anb-type">
+
+<script>
+
+function roundtripANB(str) {
+    const rule = document.styleSheets[0].cssRules[0];
+    rule.selectorText = "foo";
+    rule.selectorText = `:nth-child(${str})`;
+    // Check for parse error.
+    if(rule.selectorText == "foo") return "parse error";
+    return rule.selectorText.slice(11, -1);
+}
+function testANB(input, expected) {
+    test(()=>{
+        assert_equals(roundtripANB(input), expected);
+    }, `"${input}" becomes "${expected}"`);
+}
+
+/* Just going down all the syntax clauses one-by-one */
+//  odd | even |
+testANB("odd", "2n+1");
+testANB("even", "2n");
+//  <integer> |
+testANB("1", "1");
+testANB("+1", "1");
+testANB("-1", "-1");
+//
+//  <n-dimension> |
+testANB("5n", "5n");
+testANB("5N", "5n");
+//  '+'?† n |
+testANB("+n", "n");
+testANB("n", "n");
+testANB("N", "n");
+testANB("+ n", "parse error");
+//  -n |
+testANB("-n", "-n");
+testANB("-N", "-n");
+//
+//  <ndashdigit-dimension> |
+testANB("5n-5", "5n-5");
+//  '+'?† <ndashdigit-ident> |
+testANB("+n-5", "n-5");
+testANB("n-5", "n-5");
+testANB("+ n-5", "parse error");
+//  <dashndashdigit-ident> |
+testANB("-n-5", "-n-5");
+//
+//  <n-dimension> <signed-integer> |
+testANB("5n +5", "5n+5");
+testANB("5n -5", "5n-5");
+//  '+'?† n <signed-integer> |
+testANB("+n +5", "n+5");
+testANB("n +5", "n+5");
+testANB("+n -5", "n-5");
+testANB("+ n +5", "parse error");
+testANB("n 5", "parse error");
+//  -n <signed-integer> |
+testANB("-n +5", "-n+5");
+testANB("-n -5", "-n-5");
+testANB("-n 5", "parse error");
+//
+//  <ndash-dimension> <signless-integer> |
+testANB("5n- 5", "5n-5");
+testANB("5n- -5", "parse error");
+testANB("5n- +5", "parse error");
+testANB("-5n- 5", "-5n-5");
+//  '+'?† n- <signless-integer> |
+testANB("+n- 5", "n-5");
+testANB("n- 5", "n-5");
+testANB("+ n- 5", "parse error");
+testANB("n- +5", "parse error");
+testANB("n- -5", "parse error");
+//  -n- <signless-integer> |
+testANB("-n- 5", "-n-5");
+testANB("-n- +5", "parse error");
+testANB("-n- -5", "parse error");
+//
+//  <n-dimension> ['+' | '-'] <signless-integer>
+testANB("5n + 5", "5n+5");
+testANB("5n - 5", "5n-5");
+testANB("5n + +5", "parse error");
+testANB("5n + -5", "parse error");
+testANB("5n - +5", "parse error");
+testANB("5n - -5", "parse error");
+//  '+'?† n ['+' | '-'] <signless-integer> |
+testANB("+n + 5", "n+5");
+testANB("n + 5", "n+5");
+testANB("+ n + 5", "parse error");
+testANB("+n - 5", "n-5");
+testANB("+n + +5", "parse error");
+testANB("+n + -5", "parse error");
+testANB("+n - +5", "parse error");
+testANB("+n - -5", "parse error");
+//  -n ['+' | '-'] <signless-integer>
+testANB("-n + 5", "-n+5");
+testANB("-n - 5", "-n-5");
+testANB("-n + +5", "parse error");
+testANB("-n + -5", "parse error");
+testANB("-n - +5", "parse error");
+testANB("-n - -5", "parse error");
+
+/* Swapped ordering is invalid */
+testANB("1 - n", "parse error");
+testANB("0 - n", "parse error");
+testANB("-1 + n", "parse error");
+
+/* Odd space usage */
+testANB("2 n + 2", "parse error");
+testANB("- 2n", "parse error");
+testANB("+ 2n", "parse error");
+testANB("+2 n", "parse error");
+
+</script>