That is a very broad question, one that html5 may not be able to resolve--you might have to wait until it is an officially released language and is more fully supported to find out what its limitations are. Even then I doubt there will be any way of converting from Flash to html5. Flash involves programmed/compiled files, while html5 is merely a markup language implemented in ascii text files.
You will have to learn html5 from the ground up and then learn how to design things with it (and probably various other languages) to get the equivalent of what a Flash file can provide on its own. It is quite possible you won't be able to produce some things that Flash can. Flash has capabilities that I believe html5 alone simply will not likely be able to match.
Adobe has officially released the Flash to HTML5 converter named Wallaby. It is already downloadable for Developers to download from Adobe Labs website. Besides, Moyea SWF Decompiler is a professional SWF Decompiler that supports flash components and can easily extract flash components like images, sounds (mp3, wav), videos (flv), fonts, buttons, and even action scripts, etc. Maybe Moyea SWF Decompiler can help you .
It depends of what you have to do.
For my part, banners and stuff alike would be manageable with conversion softwares like Adobe's.
But if I would do an app for a client, I wouldn't want to put my neck and reputation on the line, for those very reasons Ned answered. I always have the mentality of programming in the native langage instead of relying on converter.
You can try with Wallaby from Adobe (http://mx.ibtimes.com/articles/10755/20110308/adobe-flash-convertir-archivos-fla-html-appl e.htm ) but it has some restrictions of what you can really export (http://labs.adobe.com/wiki/index.php/Wallaby#Installer)
Okay that is all well and good but can I use flash as an object in HTML5? I belive I can but there is a performance hit right?
For most FLV videos for example it makes sense to convert to HTML5 video formats of which I believe ogg and mpeg4 are supported but now comes the question about SWF as mentioned earlier some features are not supported via standard HTML5.
I imagine it is possible to publish SWFs and FLVs to HTML5 but by default there does not seem to be an easy way to do this. Am I wrong?
Is there a way to specify publishing output format for HTML5 instead of say:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
which it seems to default to?
Shouldn't there be a setting to change the version of HTML to HTML5 for example or any other publish page HTML version in between?
Actually playing Flash .swf files in HTML5 is simpler than in HTML 4. You can simply drag an .swf into a browser window and it will work. In HTML5 you don't need to build an OBJECT or an EMBED set of tags to contain the .swf. You can just set up a DIV and dimension it as you like to play a Flash .swf file.
Flv files will only play inside Flash, in an .swf. So if you want to play video in HTML5 you'll have to transcode the .flv to some other file container that will work in HTML5. Again the advantage is tht HTML5 is set up to play video with the OBJECT or EMBED attributions.
The document declaration for HTML5 is simply: <!DOCTYPE html><html> </html>
Everything between the two html tags gets interpreted as HTML5.