To maintain tag name case-insensitivity in HTML (when being transformed to XHTML) and to support :hover/:active, selectors are rewritten in the transformation process. Chrome and Safari have an issue where selectors are always treated as lowercase (http://jsfiddle.net/cburgmer/9k56foy5/1/, https://twitter.com/awfulben/status/534516860645941249) and thus triggers the transform process to change uppercase characters to lowercase.

It is possible that the bug exposed here just exactly mimics the current buggy Chrome & Safari behaviour.