diff --git a/include/tidyenum.h b/include/tidyenum.h
index 1889a85..24f81c4 100644
--- a/include/tidyenum.h
+++ b/include/tidyenum.h
@@ -908,6 +908,7 @@ typedef enum
         FN(PROPRIETARY_ELEMENT)           \
         FN(REPLACING_ELEMENT)             \
         FN(CUSTOM_TAG_DETECTED)           \
+        FN(CUSTOM_TAG_DETECTED_SETTING)   \
         FN(REPLACING_UNEX_ELEMENT)        \
         FN(SPACE_PRECEDING_XMLDECL)       \
         FN(SUSPECTED_MISSING_QUOTE)       \
diff --git a/src/language_en.h b/src/language_en.h
index 8fc04ba..59cae7b 100644
--- a/src/language_en.h
+++ b/src/language_en.h
@@ -386,6 +386,7 @@ static languageDefinition language_en = { whichPluralForm_en, {
     { TRIM_EMPTY_ELEMENT,           0,   "trimming empty %s"                                                       }, /* Notice */
     { REPLACING_ELEMENT,            0,   "replacing %s with %s"                                                    }, /* Notice */
     { CUSTOM_TAG_DETECTED,          0,   "detected autonomous custom tag %s; will treat as %s"                     }, /* Notice */
+    { CUSTOM_TAG_DETECTED_SETTING,  0,   "%s appears to be an autonomous custom tag; did you mean to set the custom-tags option?"  }, /* Notice */
     { TidyCustomBlocklevel,         0,   "block level"                                                             },
     { TidyCustomEmpty,              0,   "empty"                                                                   },
     { TidyCustomInline,             0,   "inline"                                                                  },
diff --git a/src/message.c b/src/message.c
index a4ca645..a5e17f7 100755
--- a/src/message.c
+++ b/src/message.c
@@ -232,6 +232,11 @@ void TY_(ReportNotice)(TidyDocImpl* doc, Node *element, Node *node, uint code)
             tagtype = tidyLocalizedString( cfg( doc, TidyUseCustomTags ) );
             message = TY_(tidyMessageCreateWithNode)(doc, element, code, TidyInfo, elemdesc, tagtype );
             break;
+
+        case CUSTOM_TAG_DETECTED_SETTING:
+            message = TY_(tidyMessageCreateWithNode)(doc, node, code, TidyInfo, nodedesc );
+            break;
+
     }
 
     messageOut( message );
diff --git a/src/tags.c b/src/tags.c
index 7965002..1c6819b 100644
--- a/src/tags.c
+++ b/src/tags.c
@@ -546,6 +546,8 @@ void show_have_html5(void)
 /* public interface for finding tag by name */
 Bool TY_(FindTag)( TidyDocImpl* doc, Node *node )
 {
+    TidyUseCustomTagsState configtype = cfg( doc, TidyUseCustomTags );
+    Bool htmlIs5 = (doc->lexer->doctype & VERS_HTML5) > 0;
     const Dict *np = NULL;
 
     if ( cfgBool(doc, TidyXmlTags) )
@@ -560,12 +562,12 @@ Bool TY_(FindTag)( TidyDocImpl* doc, Node *node )
         return yes;
     }
     
-    /* Add anonymous custom tag */
+    /* Add autonomous custom tag. This can be done in both HTML5 mode and
+       earlier, although if it's earlier we will complain about it elsewhere. */
     if ( TY_(nodeIsAutonomousCustomTag)( doc, node) )
     {
         UserTagType type;
-        TidyUseCustomTagsState configtype = cfg( doc, TidyUseCustomTags );
-        
+
         if ( configtype == TidyCustomEmpty )
             type = tagtype_empty;
         else if ( configtype == TidyCustomInline )
@@ -578,19 +580,19 @@ Bool TY_(FindTag)( TidyDocImpl* doc, Node *node )
         TY_(DeclareUserTag)( doc, TidyCustomTags, type, node->element );
         node->tag = tagsLookup(doc, &doc->tags, node->element);
 
-        if ( (doc->lexer->doctype & VERS_HTML5) )
-        {
-            TY_(ReportNotice)(doc, node, node, CUSTOM_TAG_DETECTED);
-        }
-        else
-        {
-            /* TODO: not sure whether to include this here, or let it
-               happen where it already happens; still need to suppress elsewhere */
-            TY_(ReportError)(doc, NULL, node, PROPRIETARY_ELEMENT);
-        }
-        
+        /* Output a message the first time we encounter an autonomous custom 
+           tag. This applies despite the HTML5 mode. */
+        TY_(ReportNotice)(doc, node, node, CUSTOM_TAG_DETECTED);
+
         return yes;
     }
+    else if ( TY_(nodeIsAutonomousCustomFormat)( node ) && htmlIs5 )
+    {
+        /* It looks like a custom tag, we're in HTML5, but custom-tags is
+           off, so warn the user. TODO: handle in the lexer so we don't
+           repeat this over and over again.*/
+        TY_(ReportNotice)(doc, node, node, CUSTOM_TAG_DETECTED_SETTING);
+    }
     
     return no;
 }
@@ -1054,24 +1056,30 @@ Bool nodeMatchCM( Node* node, uint contentModel )
 #endif
 
 
-/* True if the node looks like it's an autonomous custom element tag.
- */
-Bool TY_(nodeIsAutonomousCustomTag)( TidyDocImpl* doc, Node* node )
+Bool TY_(nodeIsAutonomousCustomFormat)( Node* node )
 {
-    if ( node->element && cfg( doc, TidyUseCustomTags ) != TidyCustomNo )
+    if ( node->element )
     {
         const char *ptr = strchr(node->element, '-');
-        
+
         /* Tag must contain hyphen not in first character. */
         if ( ptr && (ptr - node->element > 0) )
         {
             return yes;
         }
     }
-    
+
     return no;
 }
 
+Bool TY_(nodeIsAutonomousCustomTag)( TidyDocImpl* doc, Node* node )
+{
+    return TY_(nodeIsAutonomousCustomFormat)( node )
+            && ( cfg( doc, TidyUseCustomTags ) != TidyCustomNo );
+}
+
+
+
 /* True if any of the bits requested are set.
 */
 Bool TY_(nodeHasCM)( Node* node, uint contentModel )
diff --git a/src/tags.h b/src/tags.h
index 988ceca..0b962c6 100644
--- a/src/tags.h
+++ b/src/tags.h
@@ -134,10 +134,16 @@ Bool nodeMatchCM( Node* node, uint contentModel );
 #endif
 
 
-/* True if the node looks like it's an autonomous custom element tag.
+/* True if the node looks like it's an autonomous custom element tag. */
+Bool TY_(nodeIsAutonomousCustomFormat)( Node* node );
+
+/* True if the node looks like it's an autonomous custom element tag, and
+   TidyCustomTags is not disabled, and we're in HTML5 mode, which are all
+   requirements for valid autonomous custom tags.
 */
 Bool TY_(nodeIsAutonomousCustomTag)( TidyDocImpl* doc, Node* node );
 
+
 /* True if any of the bits requested are set.
 */
 Bool TY_(nodeHasCM)( Node* node, uint contentModel );
diff --git a/src/tidylib.c b/src/tidylib.c
index 2187d97..8b8acfd 100755
--- a/src/tidylib.c
+++ b/src/tidylib.c
@@ -1747,6 +1747,8 @@ void TY_(CheckHTMLTagsAttribsVersions)( TidyDocImpl* doc, Node* node )
     AttVal *next_attr, *attval;
     Bool attrIsProprietary = no;
     Bool attrIsMismatched = yes;
+    Bool tagLooksCustom = no;
+    Bool htmlIs5 = (doc->lexer->doctype & VERS_HTML5) > 0;
 
     while (node)
     {
@@ -1767,7 +1769,20 @@ void TY_(CheckHTMLTagsAttribsVersions)( TidyDocImpl* doc, Node* node )
                     if ( !cfgBool(doc, TidyMakeClean) ||
                         ( !nodeIsNOBR(node) && !nodeIsWBR(node) ) )
                     {
-                        TY_(ReportError)(doc, NULL, node, PROPRIETARY_ELEMENT );
+                        /* It looks custom, despite whether it's a known tag. */
+                        tagLooksCustom = TY_(nodeIsAutonomousCustomFormat)( node );
+
+                        /* If we're in HTML5 mode and the tag does not look
+                           like a valid custom tag, then issue a warning.
+                           Appearance is good enough because invalid tags have
+                           been dropped. Also, if we're not in HTML5 mode, then
+                           then everything that reaches here gets the warning.
+                           Everything else can be ignored. */
+
+                        if ( (htmlIs5 && !tagLooksCustom) || !htmlIs5 )
+                        {
+                            TY_(ReportError)(doc, NULL, node, PROPRIETARY_ELEMENT );
+                        }
 
                         if ( nodeIsLAYER(node) )
                             doc->badLayout |= USING_LAYER;