diff --git a/src/html/template/attr.go b/src/html/template/attr.go
index d65d340073..7438f51f6a 100644
--- a/src/html/template/attr.go
+++ b/src/html/template/attr.go
@@ -135,9 +135,8 @@ var attrTypeMap = map[string]contentType{
 }
 
 // attrType returns a conservative (upper-bound on authority) guess at the
-// type of the named attribute.
+// type of the lowercase named attribute.
 func attrType(name string) contentType {
-	name = strings.ToLower(name)
 	if strings.HasPrefix(name, "data-") {
 		// Strip data- so that custom attribute heuristics below are
 		// widely applied.
diff --git a/src/html/template/escape_test.go b/src/html/template/escape_test.go
index 0a6a9e49c7..43869276c0 100644
--- a/src/html/template/escape_test.go
+++ b/src/html/template/escape_test.go
@@ -1404,6 +1404,11 @@ func TestEscapeText(t *testing.T) {
 			`<script type="TEXT/JAVASCRIPT">`,
 			context{state: stateJS, element: elementScript},
 		},
+		// covering issue 19965
+		{
+			`<script TYPE="text/template">`,
+			context{state: stateText},
+		},
 		{
 			`<script type="notjs">`,
 			context{state: stateText},
diff --git a/src/html/template/transition.go b/src/html/template/transition.go
index 4a4716d782..5d34d6947e 100644
--- a/src/html/template/transition.go
+++ b/src/html/template/transition.go
@@ -106,7 +106,7 @@ func tTag(c context, s []byte) (context, int) {
 		}, len(s)
 	}
 
-	attrName := string(s[i:j])
+	attrName := strings.ToLower(string(s[i:j]))
 	if c.element == elementScript && attrName == "type" {
 		attr = attrScriptType
 	} else {