Earlier this week I needed to create some HTML prototypes for a client, and when I looked around for simple Mac OS X HTML form prototyping tools, I was disappointed to see, well, nothing.
Five to ten years ago I used to sit down with clients and create HTML form prototypes using Microsoft FrontPage, and since their 2000 version, it was actually very good. It created very clean code, kept your changes in tact, and was as simple as could be to create HTML forms.