Sorry. I just added a popup which will warn you on the homepage itself. The Design interface is very JS intensive and right now I have only tested it properly on FF. I do play to add support for all browsers very soon.
However, the finished PHP applications should run properly on all browsers.
While I guess the popup is better than nothing, seeing a window.alert anywhere usually brings up negative connotations (at least for me). I'd just add some text disclaimer on the sign up page - or at least confine the alert to that page. I tried thumbing through your site, but having to click "ok" every time I wound back on the home page didn't keep me on it for long. Best of luck to you.
However, the finished PHP applications should run properly on all browsers.