I am curious as to why sites use either doctype? more specifically I wonder what effects does this render over each other in a viewport .. even though sites for instance have no xml contained in them?
Xml vs Html doctypes
Essentially either will render your page correctly given well formed code. Even though most sites using the XHTML doctype are not using xml the doctype still renders html it's a sort of transitional point between the two standards it's better to use the XHTML flavour as it imposes certain syntax conditions on the markup that are required by XML and prompts us to code in a disciplined way and prepares us for full use of the xml standard.
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