diff --git a/content/html/document/src/nsHTMLDocument.cpp b/content/html/document/src/nsHTMLDocument.cpp
index a929aa51307d..769262fb95e6 100644
--- a/content/html/document/src/nsHTMLDocument.cpp
+++ b/content/html/document/src/nsHTMLDocument.cpp
@@ -313,11 +313,7 @@ nsHTMLDocument::CreateShell(nsPresContext* aContext,
                        aInstancePtrResult);
 }
 
-// The following Try*Charset will return false only if the charset source
-// should be considered (ie. aCharsetSource < thisCharsetSource) but we failed
-// to get the charset from this source.
-
-bool
+void
 nsHTMLDocument::TryHintCharset(nsIMarkupDocumentViewer* aMarkupDV,
                                int32_t& aCharsetSource, nsACString& aCharset)
 {
@@ -331,17 +327,17 @@ nsHTMLDocument::TryHintCharset(nsIMarkupDocumentViewer* aMarkupDV,
       aMarkupDV->SetHintCharacterSetSource((int32_t)(kCharsetUninitialized));
 
       if(requestCharsetSource <= aCharsetSource)
-        return true;
+        return;
 
-      if(NS_SUCCEEDED(rv)) {
+      if(NS_SUCCEEDED(rv) && IsAsciiCompatible(requestCharset)) {
         aCharsetSource = requestCharsetSource;
         aCharset = requestCharset;
 
-        return true;
+        return;
       }
     }
   }
-  return false;
+  return;
 }
 
 
@@ -361,6 +357,8 @@ nsHTMLDocument::TryUserForcedCharset(nsIMarkupDocumentViewer* aMarkupDV,
     rv = aMarkupDV->GetForceCharacterSet(forceCharsetFromDocShell);
   }
 
+  // Not making the IsAsciiCompatible() check here to allow the user to
+  // force UTF-16 from the menu.
   if(NS_SUCCEEDED(rv) && !forceCharsetFromDocShell.IsEmpty()) {
     aCharset = forceCharsetFromDocShell;
     //TODO: we should define appropriate constant for force charset
@@ -392,7 +390,12 @@ nsHTMLDocument::TryCacheCharset(nsICachingChannel* aCachingChannel,
 
   nsCString cachedCharset;
   rv = aCachingChannel->GetCacheTokenCachedCharset(cachedCharset);
-  if (NS_SUCCEEDED(rv) && !cachedCharset.IsEmpty())
+  // Check IsAsciiCompatible() even in the cache case, because the value
+  // might be stale and in the case of a stale charset that is not a rough
+  // ASCII superset, the parser has no way to recover.
+  if (NS_SUCCEEDED(rv) &&
+      !cachedCharset.IsEmpty() &&
+      IsAsciiCompatible(cachedCharset))
   {
     aCharset = cachedCharset;
     aCharsetSource = kCharsetFromCache;
@@ -417,69 +420,87 @@ CheckSameOrigin(nsINode* aNode1, nsINode* aNode2)
 }
 
 bool
+nsHTMLDocument::IsAsciiCompatible(const nsACString& aPreferredName)
+{
+  return !(aPreferredName.LowerCaseEqualsLiteral("utf-16") ||
+           aPreferredName.LowerCaseEqualsLiteral("utf-16be") ||
+           aPreferredName.LowerCaseEqualsLiteral("utf-16le") ||
+           aPreferredName.LowerCaseEqualsLiteral("utf-7") ||
+           aPreferredName.LowerCaseEqualsLiteral("x-imap4-modified-utf7"));
+}
+
+void
 nsHTMLDocument::TryParentCharset(nsIDocShell*  aDocShell,
                                  nsIDocument* aParentDocument,
                                  int32_t& aCharsetSource,
                                  nsACString& aCharset)
 {
-  if (aDocShell) {
-    int32_t source;
-    nsCOMPtr<nsIAtom> csAtom;
-    int32_t parentSource;
-    aDocShell->GetParentCharsetSource(&parentSource);
-    if (kCharsetFromParentForced <= parentSource)
-      source = kCharsetFromParentForced;
-    else if (kCharsetFromHintPrevDoc == parentSource) {
-      // Make sure that's OK
-      if (!aParentDocument || !CheckSameOrigin(this, aParentDocument)) {
-        return false;
-      }
-      
-      // if parent is posted doc, set this prevent autodections
-      // I'm not sure this makes much sense... but whatever.
-      source = kCharsetFromHintPrevDoc;
-    }
-    else if (kCharsetFromCache <= parentSource) {
-      // Make sure that's OK
-      if (!aParentDocument || !CheckSameOrigin(this, aParentDocument)) {
-        return false;
-      }
-
-      source = kCharsetFromParentFrame;
-    }
-    else
-      return false;
-
-    if (source < aCharsetSource)
-      return true;
-
-    aDocShell->GetParentCharset(getter_AddRefs(csAtom));
-    if (csAtom) {
-      csAtom->ToUTF8String(aCharset);
-      aCharsetSource = source;
-      return true;
-    }
+  if (!aDocShell) {
+    return;
   }
-  return false;
+  int32_t source;
+  nsCOMPtr<nsIAtom> csAtom;
+  int32_t parentSource;
+  nsAutoCString parentCharset;
+  aDocShell->GetParentCharset(getter_AddRefs(csAtom));
+  if (!csAtom) {
+    return;
+  }
+  aDocShell->GetParentCharsetSource(&parentSource);
+  csAtom->ToUTF8String(parentCharset);
+  if (kCharsetFromParentForced <= parentSource) {
+    source = kCharsetFromParentForced;
+  } else if (kCharsetFromHintPrevDoc == parentSource) {
+    // Make sure that's OK
+    if (!aParentDocument ||
+        !CheckSameOrigin(this, aParentDocument) ||
+        !IsAsciiCompatible(parentCharset)) {
+      return;
+    }
+
+    // if parent is posted doc, set this prevent autodetections
+    // I'm not sure this makes much sense... but whatever.
+    source = kCharsetFromHintPrevDoc;
+  } else if (kCharsetFromCache <= parentSource) {
+    // Make sure that's OK
+    if (!aParentDocument ||
+        !CheckSameOrigin(this, aParentDocument) ||
+        !IsAsciiCompatible(parentCharset)) {
+      return;
+    }
+
+    source = kCharsetFromParentFrame;
+  } else {
+    return;
+  }
+
+  if (source < aCharsetSource) {
+    return;
+  }
+
+  aCharset.Assign(parentCharset);
+  aCharsetSource = source;
 }
 
-bool
+void
 nsHTMLDocument::UseWeakDocTypeDefault(int32_t& aCharsetSource,
                                       nsACString& aCharset)
 {
   if (kCharsetFromWeakDocTypeDefault <= aCharsetSource)
-    return true;
-  // fallback value in case docshell return error
-  aCharset.AssignLiteral("ISO-8859-1");
+    return;
 
   const nsAdoptingCString& defCharset =
     Preferences::GetLocalizedCString("intl.charset.default");
 
-  if (!defCharset.IsEmpty()) {
+  // Don't let the user break things by setting intl.charset.default to
+  // not a rough ASCII superset
+  if (!defCharset.IsEmpty() && IsAsciiCompatible(defCharset)) {
     aCharset = defCharset;
-    aCharsetSource = kCharsetFromWeakDocTypeDefault;
+  } else {
+    aCharset.AssignLiteral("ISO-8859-1");
   }
-  return true;
+  aCharsetSource = kCharsetFromWeakDocTypeDefault;
+  return;
 }
 
 bool
@@ -494,6 +515,8 @@ nsHTMLDocument::TryDefaultCharset( nsIMarkupDocumentViewer* aMarkupDV,
   if (aMarkupDV) {
     nsresult rv =
       aMarkupDV->GetDefaultCharacterSet(defaultCharsetFromDocShell);
+    // Not making the IsAsciiCompatible() check here to allow the user to
+    // force UTF-16 from the menu.
     if(NS_SUCCEEDED(rv)) {
       aCharset = defaultCharsetFromDocShell;
 
diff --git a/content/html/document/src/nsHTMLDocument.h b/content/html/document/src/nsHTMLDocument.h
index a2e58e7c8e07..17d97ffcae3d 100644
--- a/content/html/document/src/nsHTMLDocument.h
+++ b/content/html/document/src/nsHTMLDocument.h
@@ -218,7 +218,9 @@ protected:
 
   static uint32_t gWyciwygSessionCnt;
 
-  static bool TryHintCharset(nsIMarkupDocumentViewer* aMarkupDV,
+  static bool IsAsciiCompatible(const nsACString& aPreferredName);
+
+  static void TryHintCharset(nsIMarkupDocumentViewer* aMarkupDV,
                                int32_t& aCharsetSource,
                                nsACString& aCharset);
   static bool TryUserForcedCharset(nsIMarkupDocumentViewer* aMarkupDV,
@@ -229,10 +231,10 @@ protected:
                                 int32_t& aCharsetSource,
                                 nsACString& aCharset);
   // aParentDocument could be null.
-  bool TryParentCharset(nsIDocShell*  aDocShell,
+  void TryParentCharset(nsIDocShell*  aDocShell,
                           nsIDocument* aParentDocument,
                           int32_t& charsetSource, nsACString& aCharset);
-  static bool UseWeakDocTypeDefault(int32_t& aCharsetSource,
+  static void UseWeakDocTypeDefault(int32_t& aCharsetSource,
                                       nsACString& aCharset);
   static bool TryDefaultCharset(nsIMarkupDocumentViewer* aMarkupDV,
                                   int32_t& aCharsetSource,
diff --git a/parser/html/nsHtml5MetaScannerCppSupplement.h b/parser/html/nsHtml5MetaScannerCppSupplement.h
index c6b6376432e4..402e0216ccf5 100644
--- a/parser/html/nsHtml5MetaScannerCppSupplement.h
+++ b/parser/html/nsHtml5MetaScannerCppSupplement.h
@@ -56,10 +56,7 @@ nsHtml5MetaScanner::tryCharset(nsString* charset)
       preferred.LowerCaseEqualsLiteral("utf-16be") ||
       preferred.LowerCaseEqualsLiteral("utf-16le") ||
       preferred.LowerCaseEqualsLiteral("utf-7") ||
-      preferred.LowerCaseEqualsLiteral("jis_x0212-1990") ||
-      preferred.LowerCaseEqualsLiteral("x-jis0208") ||
-      preferred.LowerCaseEqualsLiteral("x-imap4-modified-utf7") ||
-      preferred.LowerCaseEqualsLiteral("x-user-defined")) {
+      preferred.LowerCaseEqualsLiteral("x-imap4-modified-utf7")) {
     return false;
   }
   res = convManager->GetUnicodeDecoderRaw(preferred.get(), getter_AddRefs(mUnicodeDecoder));
diff --git a/parser/html/nsHtml5StreamParser.cpp b/parser/html/nsHtml5StreamParser.cpp
index c8671bcdde58..1ac7f733bcf2 100644
--- a/parser/html/nsHtml5StreamParser.cpp
+++ b/parser/html/nsHtml5StreamParser.cpp
@@ -1213,10 +1213,7 @@ nsHtml5StreamParser::PreferredForInternalEncodingDecl(nsACString& aEncoding)
       preferred.LowerCaseEqualsLiteral("utf-16be") ||
       preferred.LowerCaseEqualsLiteral("utf-16le") ||
       preferred.LowerCaseEqualsLiteral("utf-7") ||
-      preferred.LowerCaseEqualsLiteral("jis_x0212-1990") ||
-      preferred.LowerCaseEqualsLiteral("x-jis0208") ||
-      preferred.LowerCaseEqualsLiteral("x-imap4-modified-utf7") ||
-      preferred.LowerCaseEqualsLiteral("x-user-defined")) {
+      preferred.LowerCaseEqualsLiteral("x-imap4-modified-utf7")) {
     // Not a rough ASCII superset
     mTreeBuilder->MaybeComplainAboutCharset("EncMetaNonRoughSuperset",
                                             true,
diff --git a/parser/htmlparser/tests/reftest/bug599320-1-ref.html b/parser/htmlparser/tests/reftest/bug599320-1-ref.html
new file mode 100644
index 000000000000..bb48fe5d256b
--- /dev/null
+++ b/parser/htmlparser/tests/reftest/bug599320-1-ref.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+<meta charset=utf-8>
+<meta content="width=device-width, initial-scale=1" name="viewport">
+<title>UTF-16 doc</title>
+</head>
+<body>
+<h1>UTF-16 doc</h1>
+
+<p>Euro sign: €</p>
+<p>iframe:</p>
+<iframe src=frame599320-1-ref.html width=300 height=400></iframe>
+
+</body>
+</html>
+
diff --git a/parser/htmlparser/tests/reftest/bug599320-1.html b/parser/htmlparser/tests/reftest/bug599320-1.html
new file mode 100644
index 000000000000..590e9126c3fa
Binary files /dev/null and b/parser/htmlparser/tests/reftest/bug599320-1.html differ
diff --git a/parser/htmlparser/tests/reftest/frame599320-1-ref.html b/parser/htmlparser/tests/reftest/frame599320-1-ref.html
new file mode 100644
index 000000000000..735c368f8b53
--- /dev/null
+++ b/parser/htmlparser/tests/reftest/frame599320-1-ref.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+<meta charset=utf-8>
+<meta content="width=device-width, initial-scale=1" name="viewport">
+<title>Non-UTF-16 doc</title>
+</head>
+<body>
+<h1>Non-UTF-16 doc</h1>
+
+<p>Euro sign: €</p>
+
+</body>
+</html>
+
diff --git a/parser/htmlparser/tests/reftest/frame599320-1.html b/parser/htmlparser/tests/reftest/frame599320-1.html
new file mode 100644
index 000000000000..2b5b3383690e
--- /dev/null
+++ b/parser/htmlparser/tests/reftest/frame599320-1.html
@@ -0,0 +1,1092 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+<!-- More than 1 KB of space -->
+
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+
+<meta charset=utf-8>
+<meta content="width=device-width, initial-scale=1" name="viewport">
+<title>Non-UTF-16 doc</title>
+</head>
+<body>
+<h1>Non-UTF-16 doc</h1>
+
+<p>Euro sign: €</p>
+<script>
+window.onload = function() {
+  window.mozRequestAnimationFrame(function() {
+    parent.document.documentElement.removeAttribute("class");
+  });
+}
+</script>
+</body>
+</html>
+
diff --git a/parser/htmlparser/tests/reftest/reftest.list b/parser/htmlparser/tests/reftest/reftest.list
index cd378313cbf1..d4015f890bd8 100644
--- a/parser/htmlparser/tests/reftest/reftest.list
+++ b/parser/htmlparser/tests/reftest/reftest.list
@@ -5,6 +5,7 @@
 == bug582788-1.html bug582788-1-ref.html
 == bug582940-1.html bug582940-1-ref.html
 == bug592656-1.html bug592656-1-ref.html
+== bug599320-1.html bug599320-1-ref.html
 == bug608373-1.html bug608373-1-ref.html
 fails-if(/^Windows\x20NT\x206\.1/.test(http.oscpu)&&!layersGPUAccelerated&&!azureSkia) == view-source:bug482921-1.html bug482921-1-ref.html # bug 703201
 == view-source:bug482921-2.xhtml bug482921-2-ref.html