--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -1727,45 +1727,61 @@ nsGenericHTMLElement::GetURIListAttr(nsI
 
   nsAutoString value;
   if (!GetAttr(kNameSpaceID_None, aAttr, value))
     return NS_OK;
 
   nsIDocument* doc = OwnerDoc(); 
   nsCOMPtr<nsIURI> baseURI = GetBaseURI();
 
-  // Value contains relative URIs split on spaces (U+0020)
-  const char16_t *start = value.BeginReading();
-  const char16_t *end   = value.EndReading();
-  const char16_t *iter  = start;
-  for (;;) {
-    if (iter < end && *iter != ' ') {
+  nsString::const_iterator end;
+  value.EndReading(end);
+
+  nsAString::const_iterator iter;
+  value.BeginReading(iter);
+
+  while (iter != end) {
+    while (nsCRT::IsAsciiSpace(*iter) && iter != end) {
+      ++iter;
+    }
+
+    if (iter == end) {
+      break;
+    }
+
+    nsAString::const_iterator start = iter;
+
+    while (iter != end && !nsCRT::IsAsciiSpace(*iter)) {
       ++iter;
-    } else {  // iter is pointing at either end or a space
-      while (*start == ' ' && start < iter)
-        ++start;
-      if (iter != start) {
-        if (!aResult.IsEmpty())
-          aResult.Append(char16_t(' '));
-        const nsSubstring& uriPart = Substring(start, iter);
-        nsCOMPtr<nsIURI> attrURI;
-        nsContentUtils::NewURIWithDocumentCharset(getter_AddRefs(attrURI),
-                                                  uriPart, doc, baseURI);
-        if (attrURI) {
-          nsAutoCString spec;
-          attrURI->GetSpec(spec);
-          AppendUTF8toUTF16(spec, aResult);
-        } else {
-          aResult.Append(uriPart);
-        }
-      }
-      start = iter = iter + 1;
-      if (iter >= end)
-        break;
+    }
+
+    if (!aResult.IsEmpty()) {
+      aResult.Append(NS_LITERAL_STRING(" "));
     }
+
+    const nsSubstring& uriPart = Substring(start, iter);
+    nsCOMPtr<nsIURI> attrURI;
+    nsresult rv =
+      nsContentUtils::NewURIWithDocumentCharset(getter_AddRefs(attrURI),
+                                                uriPart, doc, baseURI);
+    if (NS_FAILED(rv)) {
+      aResult.Append(uriPart);
+      continue;
+    }
+
+    MOZ_ASSERT(attrURI);
+
+    nsAutoCString spec;
+    rv = attrURI->GetSpec(spec);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      aResult.Append(uriPart);
+      continue;
+    }
+
+    AppendUTF8toUTF16(spec, aResult);
   }
 
   return NS_OK;
 }
 
 HTMLMenuElement*
 nsGenericHTMLElement::GetContextMenu() const
 {