See #1522, #1112, #1059 and #335.

Okay, do as you please but I will say that the uppercase doctype was required by xhtml and the lowercase version hightlights in red in many text-editors. I see no reason not to change it other than wanting the entire document to be lowercase which it isn't anyway.

There's a lot of discussion in those previous issues. It's worth reading through, especially since the change back to lowercase was just last month.

All the other tags and attributes are lowercase though, personally I like it much more.
It sounds more like a problem with your editor than anything else, but it's true that it still tends to be that way and used to be the standard; Haml, Slim and Jade all return uppercase.

I don't think we can say much that hasn't been said already, it's NEVER mattered either way. One is a tradition, the other is consistent with all the other tags - unless you want to use uppercase for everything :P

many text-editors

@brianmillar9 can you be more specific? Thanks.

some IDEs and HTML Editors

@brianmillar9 thanks for the pull request, but will be sticking with lowercase. The highlighting problem represents a bug in the IDE/text editor, and thus, it should be fixed by them (if they haven't already)