emmetio/emmet — pull request #275 by elmimmo (2013-11-16 10:01:20+00:00)
Title: Make HTML5 "DOCTYPE" uppercase for XML validity

While the HTML spec states that "DOCTYPE" in HTML5 is case insensitive, it still must be uppercase in XHTML5 for proper XML validation.

Using it in lowercase, then, brings not value over doing so in uppercase, and instead introduces potential problems.