While duckduckgo.com is using the standard HTML5 doctype, html.duckduckgo.com is...

While duckduckgo.com is using the standard HTML5 doctype, html.duckduckgo.com is using

  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

and lite.duckduckgo.com is using

  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

Is this on purpose? To make the website work on older browsers?