Nitro HTML Pro and Nitro HTML permit to edit HTML source files, and visualize how the resulting HTML page will be displayed in compliant Web browsers.
The source code can optionally contain JavaScript and SVG specifications.
Nitro HTML has been mentioned during an episode of The Mac Observer & Mac Geek Gab podcast in January 2013. You can see the show notes and download the show or subscribe to their podcasts in iTunes.
There is also a Russian overview of the application Nitro HTML published on August 18, 2012 on YouTube.