Invalid chars should not be allowed as first character in custom element name

Source/core/dom/custom/CustomElement.cpp

Index: Source/core/dom/custom/CustomElement.cpp
diff --git a/Source/core/dom/custom/CustomElement.cpp b/Source/core/dom/custom/CustomElement.cpp
index 4253c5fea893093d0354146864b2c070599e7442..e526bff26b43e5c958a0bc9a40ef3fcbf1927bf0 100644
--- a/Source/core/dom/custom/CustomElement.cpp
+++ b/Source/core/dom/custom/CustomElement.cpp
@@ -66,6 +66,24 @@ void CustomElement::addEmbedderCustomElementName(const AtomicString& name)
     embedderCustomElementNames().append(lower);
 }
 
+static inline bool isValidNCName(const AtomicString& name)
+{
+    if (kNotFound != name.find(':'))
+        return false;
+
+    if (!name.string().is8Bit()) {
+        const UChar32 c = name.characters16()[0];
+        // These characters comes under CombiningChar in NCName and according to
+        // NCName only BaseChar and Ideodgraphic can come as first chars.
+        // Also these characters come under Letter_Other in UnicodeData, thats
+        // why they pass as valid document name.
+        if (c == 0x0B83 || c == 0x0F88 || c == 0x0F89 || c == 0x0F8A || c == 0x0F8B)
+            return false;
+    }
+
+    return Document::isValidName(name.string());
+}
+
 bool CustomElement::isValidName(const AtomicString& name, NameSet validNames)
 {
     if ((validNames & EmbedderNames) && kNotFound != embedderCustomElementNames().find(name))
@@ -79,7 +97,7 @@ bool CustomElement::isValidName(const AtomicString& name, NameSet validNames)
         }
 
         if (kNotFound == reservedNames.find(name))
-            return Document::isValidName(name.string());
+            return isValidNCName(name);
     }
 
     return false;