The document type declaration is required for your theme. This is certainly one of the easiest steps in creating a theme!

The theme engine loads the core/doctype.php file in your theme directory and expects to find the theme_page_doctype hook defined. Your hook should return the doctype string.

The following returns the old 4.01 transitional doctype.

function theme_page_doctype()
{
    return "\n";
}

A more modern HTML 5 example:

function theme_page_doctype()
{
    return "\n";
}