Why do you need that HTML 5?
I have made a simple course in flash, it plays sound and shows texts then you can evaluate if it was correct or not. It uses general objects as question steps so it can be re-used to practice calculus, geography for kids or language study for adults. Its sort of a flashcard app but with video and sound (if supported by browser) and will have several question evaluation classes (like self evaluation "I was wrong/right", fill in number, date, text, multiple choice)
Before spending more time programming more classes for the course app I want to check what platform would be best to release it on. HTML 5 seemed to me the best choice since it is available for mobile phones, windows, mac and linux. Java or flash are less desired options since it requires the user to install and configure extra software.
Especially mobile phones would be nice since you can practice your vocab while commuting or waiting for something.
Here come the trouble though, opera mobile not yet supports audio tag. It might support Audio object but doesn't play anything and it will not support the html5 local storage for local files. It doesn't even allow cookies from local files. So when audio will be supported saving state is still not possible. IE has been a disaster, where I have minimal functionality working in firefox and opera IE just does it's utter best not to make it work.
But for all the small problems that the browsers will have, at least it is available with limited but soon to be expanded functionality where I am afraid with the recent bashing against flash that this might be less and less supported. Android will have/has a good html5 browser, I think iphone has good support, opera mobile can be used for windows phones (without sound or saving state) and firefox for any desk/laptop.