registration-page does not work with IE8

I have everything up and running untill u use IE8 the “submit-button” becomes a “clear-all-button”. The not-required fields ae swept clean and the required filds stay as they are, nothing else happens. No comfirmationtext, no conformationmail and no account created. Does anyone know how to solve this?

https://www.mijn.speelplaats.org
P.s. I only have IE8 I don’t know what IE9 does yet.