I have not yet written any HTML5 but I have many many years experience (not all happy) of working with the previous versions. My understanding is that the current browser support is still quite patchy. In my organisation we still have users on XP and IE6!
If I were to create a new site using HTML5, how much of my time would be spend handling all the differences between browsers?
How could I let the page degrade gracefully when some features are not available?
Does your book also handle these issues.
Your organization needs to get off IE6. Time to bite the bullet, spend the money and move. If your company does not control the IE6, start dropping support. Microsoft is also starting to force upgrade IE7 so there will no longer be that browser to deal with.
I know you probably have no control in this matter, but dropping support starts at a developer level. Our developers brought facts to the management saying we spend XX hours a week fixing bugs for a browser that has Y% share. That means we are spending $ZZZ dollars per user.
How will HTML5 work with browsers that do not support it? It depends on what you are using. Input elements will degrade to plain textboxes, canvas stuff will not work out of the box, some of the CSS stuff will not apply. The site will still funciton for the most part, it will not be as pretty/useful.