The core/meta.php can be used to add any <meta> tags required in your theme.
The theme engine loads the core/meta.php file in your theme directory and expects to find the theme_page_meta_tags hook defined. Your hook should return any <meta> tags that you require.
The following returns the old 4.01 transitional doctype.
function theme_page_meta_tags() { return " <!-- Meta tags --> <meta charset='utf-8'> <meta name='viewport' content='width=device-width, initial-scale=1.0'> "; }