--- idx.html	2016-02-04 06:14:14.000000000 +0100
+++ idx.html	2017-08-11 13:22:35.678945500 +0200
@@ -81,6 +81,7 @@
 proc BeginHeader {} {
     global la ti
     upvar 1 lines lines
+    lappend lines [markup {<!DOCTYPE html>}]
     lappend lines [markup <html>]
     lappend lines [ht_comment [c_provenance]]
     lappend lines [ht_comment "$la"]
