Custom css attributes
elm
Solution
CSS in general
Elm tries to abstract over things like HTML and CSS. You can find a few discussion on the mailing-list, some people literally asking to add support for strings with CSS. That doesn't mean that Elm is as good at styling webpages as CSS3 is yet. There's more to be done, so if you know CSS and know what you want, maybe you can contribute to making Elm's Graphics API better.
Transitions
Transitions really something you can (/should IMHO) do within Elm. Notice there are a few community libraries available for Elm, among which is a simple Easing library. That might help you on your way. If you don't know where to start, let me know and I'll write you an example.
Problem
I'm slightly confused about what css attributes can be added to an element. I see that in Graphics.Element there are width, height, size, etc which I can add using these functions, but how can I add other attributes that are not in this module? For example, I'm currently looking for some way to add css3-transitions. Is there a function for specifying custom style attributes? Update: adding custom css/html attributes is possible now with elm-html library.