diff --git a/components/gfx/tests/text_util.rs b/components/gfx/tests/text_util.rs
index 2a973cd23110..cb5a1d5c2532 100644
--- a/components/gfx/tests/text_util.rs
+++ b/components/gfx/tests/text_util.rs
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-use gfx::text::util::{transform_text, CompressionMode};
+use gfx::text::util::{is_cjk, transform_text, CompressionMode};
 
 #[test]
 fn test_transform_compress_none() {
@@ -104,3 +104,22 @@ fn test_transform_compress_whitespace_newline_no_incoming() {
         assert_eq!(trimmed_str, oracle)
     }
 }
+
+#[test]
+fn test_is_cjk() {
+    // Test characters from different CJK blocks
+    assert_eq!(is_cjk('〇'), true);
+    assert_eq!(is_cjk('㐀'), true);
+    assert_eq!(is_cjk('あ'), true);
+    assert_eq!(is_cjk('ア'), true);
+    assert_eq!(is_cjk('㆒'), true);
+    assert_eq!(is_cjk('ㆣ'), true);
+    assert_eq!(is_cjk('龥'), true);
+    assert_eq!(is_cjk('𰾑'), true);
+    assert_eq!(is_cjk('𰻝'), true);
+
+    // Test characters from outside CJK blocks
+    assert_eq!(is_cjk('a'), false);
+    assert_eq!(is_cjk('🙂'), false);
+    assert_eq!(is_cjk('©'), false);
+}
diff --git a/components/gfx/text/util.rs b/components/gfx/text/util.rs
index be9ba144dca7..016e18f2b563 100644
--- a/components/gfx/text/util.rs
+++ b/components/gfx/text/util.rs
@@ -149,5 +149,6 @@ pub fn is_cjk(codepoint: char) -> bool {
     }
 
     // https://en.wikipedia.org/wiki/Plane_(Unicode)#Supplementary_Ideographic_Plane
-    unicode_plane(codepoint) == 2
+    // https://en.wikipedia.org/wiki/Plane_(Unicode)#Tertiary_Ideographic_Plane
+    unicode_plane(codepoint) == 2 || unicode_plane(codepoint) == 3
 }