--- a/parser/html/javasrc/TreeBuilder.java
+++ b/parser/html/javasrc/TreeBuilder.java
@@ -1162,16 +1162,17 @@ public abstract class TreeBuilder<T> imp
                                     flushCharacters();
                                     reconstructTheActiveFormattingElements();
                                     /*
                                      * Append the token's character to the
                                      * current node.
                                      */
                                     continue;
                             }
+                            // CPPONLY: MOZ_FALLTHROUGH_ASSERT();
                         default:
                             /*
                              * A character token that is not one of one of
                              * U+0009 CHARACTER TABULATION, U+000A LINE FEED
                              * (LF), U+000C FORM FEED (FF), or U+0020 SPACE
                              */
                             switch (mode) {
                                 case INITIAL:

--- a/parser/html/nsHtml5TreeBuilder.cpp
+++ b/parser/html/nsHtml5TreeBuilder.cpp
@@ -394,16 +394,17 @@ nsHtml5TreeBuilder::characters(const cha
                   accumulateCharacters(buf, start, i - start);
                   start = i;
                 }
                 flushCharacters();
                 reconstructTheActiveFormattingElements();
                 continue;
               }
             }
+            MOZ_FALLTHROUGH_ASSERT();
           }
           default: {
             switch (mode) {
               case INITIAL: {
                 documentModeInternal(QUIRKS_MODE, nullptr, nullptr, false);
                 mode = BEFORE_HTML;
                 i--;
                 continue;