Add doctype to xsl document

I'm curious about what was complaining about the doctype not found? A certain browser? A certain validator?

I get a 'no doctype found' warning when running my site http://toyrider.com/sitemap.xml through the the W3C Validator at https://validator.w3.org

After applying the patch, deleting the old sitemap.xml, rebuilding a new one and clearing caches I still don't get a doctype? This time the sitemap is at http://best-table-tennis-tables.com/sitemap.xml

Here is the document head:
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="//best-table-tennis-tables.com/sitemap.xsl"?>

Gotcha.

It looks like based on http://stackoverflow.com/questions/3387127/set-html5-doctype-with-xslt that we actually might want to use the following instead?

<xsl:output method="html" doctype-system="about:legacy-compat" encoding="UTF-8" indent="yes" />

Updated patch with Dave's xsl suggestion from Stack Overflow.

W3 gives a passing grade to my very simple sitemap test.

Thanks, tested and committed #5 to all branches!