I am still in the learning curve of webdesign, i suppose i will never stop. But today i realised, after being unable to use my own pc how shoddy my websites are in other browers, i.e. not firefox

Functions, javascript coding, css and basic html fields just don't work and the sites don't feel the same as what i want.Im just wondering if anyone has any tips on how to make your website function in all browsers..
I mean i always try and check each browser durig coding, i make sure my page fully validates, css and xhtml. What else can i do? Other than refuse all browsers to my site but firefox :P