The CSS style is destroyed when you use submit button in "CSS Styles" page, on geoserver 2.6-SNAPSHOT
Steps to reproduce :
Create an CSS Style using styles page and format "CSS".
Goto page "CSS Styles" (left bottom link on UI)
click on "Edit a different style"
select your css style previously created.
press "submit" button.
return to styles page, and check your style. It has been converted to SLD, but format still CSS, and then it becomes unusable.