The <link> to the stylesheet should include the type attribute.
diff --git a/Doc/perl/l2hinit.perl b/Doc/perl/l2hinit.perl
index 7a3aeeb..0bc9aa5 100644
--- a/Doc/perl/l2hinit.perl
+++ b/Doc/perl/l2hinit.perl
@@ -616,7 +616,7 @@
. "charset=$CHARSET\">\n")
: ''),
($BASE ? "<base href=\"$BASE\">\n" : ''),
- "<link rel=\"STYLESHEET\" href=\"$STYLESHEET\">\n",
+ "<link rel=\"STYLESHEET\" href=\"$STYLESHEET\" type='text/css'>\n",
"<link rel=\"first\" href=\"$FILE.html\">\n",
($HAVE_TABLE_OF_CONTENTS
? ('<link rel="contents" href="contents.html" title="Contents">'