- Backwards compatibility. For example would my spanking new HTML5 compliant page work on current browser versions? From what I've read so far current browsers are designed to ignore tags that they don't recognise so this helps in designing for old browsers. I am confident browsers will be updated to suit but the user base is a whole other story.It might probably take a while for HTML5 to become mainstream thus giving everyone time to get on board.
- What has been added, deprecated and removed? One main part of HTML5 is that it defines several scripting APIs as part of the language as opposed to being just markup which is better suited for web application platforms. APIs are defined for mulitmedia playback, file storage and document editing among other things. One of the aims of HTML5 is to reduce the dependancy on multimedia browser add-ons like Adobe Flash and Sliverlight. So now the <audio> and<video> tags have new functionality. Also some tags like <font> and <center> have been removed with the reasoning being that these effects can be achieved through CSS. PUT and DELETE methods have been added to the form element which makes it easier to comply with REST.
- Upgrade and conversion procedures. What can and should I do with existing HTML4 compliant webpages? I believe this has to be done on a case by case basis paying special attention to priority, available conversion tools, testing strategy, timeline and benefits analysis. For new applications or additions the decision is easier and the tendency would be to go with the newer technology. And of course if you use a third party CMS or some other layer of web technology the upgrade to HTML5 will be more or less transparent to you.
If you want to see HTML5 in action you can try out the YouTube HTML5 video player.
So there you have it, a gist of what HTML5 is all about from my perspective. If you have more interest in this you can checkout the entire specification as a work in progress here.