Well, if you just started a career in UII developing, you should probably know what their responsibilities are by now.
Here is a little info on a UI developer: http://programmers.stackexchange.com/questions/58246/who-can-call-themselves-a-ui-developer
I know that i read somewhere where a ui developer should know basically any web language.
I found this to help you answer your original question about html5 and IE:
http://www.pcpro.co.uk/blogs/2011/03/07/making-html5-work-in-old-versions-of-internet-explorer/