Newsletter signup and success notification challenge demo

In general I tried to use semantic HTML tags for the basic structure. The first signup page is a section and the notification is an aside element that's hidden initially. I wrote the script code using plain vanilla javascript without any framework. Also, I use preventDefault to stop the submission of the form, so no backend connection is made. The javascript is loaded on the window.onload function and it's separated into 3 sections: elements/document objects, handlers and finally events. I think this generic structure works fine for small implementations like this. Any recommendation is welcome. Thanks!