--- a/parser/html/javasrc/TreeBuilder.java
+++ b/parser/html/javasrc/TreeBuilder.java
@@ -3293,27 +3293,30 @@ public abstract class TreeBuilder<T> imp
                         case ';':
                             end = i;
                             break charsetloop;
                         default:
                             continue;
                     }
             }
         }
-        String charset = null;
         if (start != -1) {
             if (end == -1) {
-                end = buffer.length;
+                if (charsetState == CHARSET_UNQUOTED) {
+                    end = buffer.length;
+                } else {
+                    return null;
+                }
             }
-            charset = Portability.newStringFromBuffer(buffer, start, end
+            return Portability.newStringFromBuffer(buffer, start, end
                     - start
                 // CPPONLY: , tb, false
             );
         }
-        return charset;
+        return null;
     }
 
     private void checkMetaCharset(HtmlAttributes attributes)
             throws SAXException {
         String charset = attributes.getValue(AttributeName.CHARSET);
         if (charset != null) {
             if (tokenizer.internalEncodingDeclaration(charset)) {
                 requestSuspension();

--- a/parser/html/nsHtml5TreeBuilder.cpp
+++ b/parser/html/nsHtml5TreeBuilder.cpp
@@ -2337,25 +2337,28 @@ nsHtml5TreeBuilder::extractCharsetFromCo
           default: {
             continue;
           }
         }
       }
     }
   }
 charsetloop_end:;
-  nsHtml5String charset = nullptr;
   if (start != -1) {
     if (end == -1) {
-      end = buffer.length;
+      if (charsetState == CHARSET_UNQUOTED) {
+        end = buffer.length;
+      } else {
+        return nullptr;
+      }
     }
-    charset = nsHtml5Portability::newStringFromBuffer(
+    return nsHtml5Portability::newStringFromBuffer(
       buffer, start, end - start, tb, false);
   }
-  return charset;
+  return nullptr;
 }
 
 void
 nsHtml5TreeBuilder::checkMetaCharset(nsHtml5HtmlAttributes* attributes)
 {
   nsHtml5String charset =
     attributes->getValue(nsHtml5AttributeName::ATTR_CHARSET);
   if (charset) {