Neither JavaScript or Flash are allowed in your customized styles because they can involve security problems.
Since users can log in to InsaneJournal and have their username/password saved, if we allowed either JavaScript or Flash, people could write scripts to intercept the username/password and save it, or e-mail it to the malicious script author. We've looked into solutions to allow JavaScript and Flash, but there seems to be no good way to prevent them from having access to the cookies on your computer which store this information.
If you know of a robust way for us to allow JavaScript or Flash, please let us know! For now, however, all JavaScript and Flash is removed from journal styles and overrides before it's sent to any browser.