All are valid workflows, depending on the designer and developer's preference. For those who have extensive experience in Adobe Flash, there is hope in that you have that extensive Actionscript 3 knowledge.
There are experimental technologies out there as well, with Google having a team dedicated to a new converter technology call Swiffy which basically is an online tool that converts Flash files to an animated HTML equivalent. It shows that Google, and its' vast online advertising network, is also currently working on an actual "working" solution for this conversion process we're all finding ourselves in. There is even a Flash CS4+ extension that can be downloaded to add an option within the program itself to "export Flash as HTML".
Swiffy currently only supports Actionscript 2 based animations however, as well as only successful for simple, timeline based ad banners. Though Google seems to at least be very aware of this and is working on a full solution.
Adobe Edge Preview 4 was released just recently which allows a very similar experience to creating objects, and adding symbols like in Flash. Add to that with Web Fonts, and soon it may emerge as a great tool for creating HTML5 based animations coming soon in the future.
There are some other tools out there like Hippo HTML5 Animator and Tumult Hype.
A personal routine as of late is to first create banner ads in Flash, using superior animation effects and filters in Greensock and Actionscript code, then convert them to an HTML and iFrame equivalent. This conversion step is amazingly very quickly done, using similar animation methods using jQuery code and with tools like the new Adobe Edge.