You can not use JavaScript or Flash in your customized styles because it is a security problem.

Since users can log in to LiveJournal 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 email 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, though, all JavaScript and all Flash is removed from journal styles and overrides before it's sent to any browser.