The first question is determined by your target audience. For example if you were in China you might not want to go HTML 5. Why? Well it's pretty clear if you do YOUR RESEARCH that China leads in XP use.
http://en.wikipedia.org/wiki/Usage_share_of_operating_systems is also telling.
As to which browser? I fear you are not using the web to do your research and for you, my answer is you should not do this.
I like HTML 5 but for your questions it only tells me you are not leveraging what research has been done and should go with what the boss asked for.
Can we use HTML 5 now? Which browser support HTML 5 most? Please, somebody help me.