If you visit a site that uses Flash, and you don’t have Flash installed on your browser, a message will be shown to you to install Flash. A similar procedure happens when Silverlight is involved instead of Flash.
What happens if a browser does not support HTML 5? Will I have to install something to get HTML 5 support in that case? Is that even possible?
Browsers will ignore elements it doesn’t support and it won’t apply css styles to those elements either. You won’t get any explicit message that the browser doesn’t support HTML5. You can, as the page author, provide your own message if the browser doesn’t depending on the circumstance. You can, for example, provide a message that will display to the user inside a video or audio tag.