Is tag case sensitive? The simulator here in code academy gave an error when I typed but I have read in multiple places that html is case insensitivity.

HTML5 is meant to be able to comply with XML the way its precursor, XHTML does. XML is case sensitive and the rule in XHTML is that all tags are written in lowercase.

HTML 4 and earlier are not case sensitive, and HTML5 follows the specs of HTML when it is served as text/html, so is likewise, not case sensitive. We may use upper of lower case tags, though following the adopted convention is suggested in most circles.