HTML5 is not official and mandated version of HTML and not every browser vendor supports it. You need to be aware of your users and how many of them using browser that supports HTML5. Here is a break down and browsers currently supporting HTML5 and you need to be aware.

• Chrome – version 1 and up
• Firefox – version 1.5 and up
• Internet Explorer – version 9 and up
• Opera  - version 9 and up
• Safari – version 1.3 and up

In case browser you are running your application on does not support HTML5 you can always fallback on the 

<canvas>   Change your browser version to support HTML5. </canvas>

Canvas is somewhat limited in terms of alternatives for items residing inside canvas. For example, broken images cannot have alt tag as in regular img tag. While designing your canvas you have to be aware of this issues.