diff --git a/lib/model/katex.dart b/lib/model/katex.dart
index fa724d1267..64c5eea82b 100644
--- a/lib/model/katex.dart
+++ b/lib/model/katex.dart
@@ -357,8 +357,19 @@ class _KatexParser {
         case 'mop':
         case 'mclose':
         case 'minner':
+        case 'mbin':
+        case 'mpunct':
+        case 'nobreak':
+        case 'allowbreak':
+        case 'mathdefault':
           // Ignore these classes because they don't have a CSS definition
           // in katex.scss, but we encounter them in the generated HTML.
+          // (Why are they there if they're not used?  The story seems to be:
+          // they were used in KaTeX's CSS in the past, before 2020 or so; and
+          // they're still used internally by KaTeX in producing the HTML.
+          //   https://github.com/KaTeX/KaTeX/issues/2194#issuecomment-584703052
+          //   https://github.com/KaTeX/KaTeX/issues/3344
+          // )
           break;
 
         default: