Extensible HyperText Markup Language – Wikipedia を参考にして、テンプレートを用意してみました。
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="ja" xml:lang="ja">
<head>
<meta http-equiv="Content-Type"
content="application/xhtml+xml; charset=UTF-8" />
<title>HTML title</title>
</head>
<body>
</body>
</html>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="ja" xml:lang="ja">
<head>
<meta http-equiv="Content-Type"
content="application/xhtml+xml; charset=UTF-8" />
<title>HTML title</title>
</head>
<body>
</body>
</html>