diff --git a/testing/web-platform/tests/css/css-content/parsing/content-valid.html b/testing/web-platform/tests/css/css-content/parsing/content-valid.html
index 097bf66f62b2..80bb54f8dcb3 100644
--- a/testing/web-platform/tests/css/css-content/parsing/content-valid.html
+++ b/testing/web-platform/tests/css/css-content/parsing/content-valid.html
@@ -47,6 +47,9 @@ test_valid_value_combinations("content", `counters(counter-name, ".") "potato"`)
 test_valid_value_combinations("content", `"(" counters(counter-name, ".", counter-style) ")"`);
 test_valid_value_combinations("content", `open-quote "hello" "world" close-quote`);
 test_valid_value_combinations("content", `url("picture.svg") "hello"`);
+
+test_valid_value_combinations("content", "counter(\\})");
+test_valid_value_combinations("content", `counters(\\}, ".")`);
 </script>
 </body>
 </html>
diff --git a/testing/web-platform/tests/css/css-lists/parsing/counter-increment-valid.html b/testing/web-platform/tests/css/css-lists/parsing/counter-increment-valid.html
index 0187622339d8..557290f87005 100644
--- a/testing/web-platform/tests/css/css-lists/parsing/counter-increment-valid.html
+++ b/testing/web-platform/tests/css/css-lists/parsing/counter-increment-valid.html
@@ -20,6 +20,7 @@ test_valid_value('counter-increment', 'section calc(-2.5)');
 test_valid_value('counter-increment', 'section calc(10 + (5 * sign(2cqw - 10px)))');
 test_valid_value('counter-increment', 'a 1 b 2 c 3 d 4 e 5');
 test_valid_value('counter-increment', 'a 1 b 2 c 3 a 4 b 5');
+test_valid_value('counter-increment', 'a\\ 8 9');
 </script>
 </body>
 </html>
diff --git a/testing/web-platform/tests/css/css-lists/parsing/counter-reset-valid.html b/testing/web-platform/tests/css/css-lists/parsing/counter-reset-valid.html
index 4e462919edbe..55c9f5e4e394 100644
--- a/testing/web-platform/tests/css/css-lists/parsing/counter-reset-valid.html
+++ b/testing/web-platform/tests/css/css-lists/parsing/counter-reset-valid.html
@@ -26,6 +26,7 @@ test_valid_value('counter-reset', 'section calc(-2.5)');
 test_valid_value('counter-reset', 'section calc(10 + (5 * sign(2cqw - 10px)))');
 test_valid_value('counter-reset', 'a 1 b 2 c 3 d 4 e 5');
 test_valid_value('counter-reset', 'a 1 b 2 c 3 a 4 b 5');
+test_valid_value('counter-reset', 'a\\ 8 9');
 </script>
 </body>
 </html>
diff --git a/testing/web-platform/tests/css/css-lists/parsing/counter-set-valid.html b/testing/web-platform/tests/css/css-lists/parsing/counter-set-valid.html
index f7f1c661296a..e985fcae149f 100644
--- a/testing/web-platform/tests/css/css-lists/parsing/counter-set-valid.html
+++ b/testing/web-platform/tests/css/css-lists/parsing/counter-set-valid.html
@@ -20,6 +20,7 @@ test_valid_value('counter-set', 'section calc(-2.5)');
 test_valid_value('counter-set', 'section calc(10 + (5 * sign(2cqw - 10px)))');
 test_valid_value('counter-set', 'a 1 b 2 c 3 d 4 e 5');
 test_valid_value('counter-set', 'a 1 b 2 c 3 a 4 b 5');
+test_valid_value('counter-set', 'a\\ 8 9');
 </script>
 </body>
 </html>