Hey! I watched the full video and clicked around your site a bit. I think this is fantastic. It's just anecdotal, but I've met quite a few people who are far more comfortable in HTML/CSS than they are in any backend work, so although this doesn't count as market validation, I don't think you're targeting too small of a segment.
In fact, I had someone pick my brain just a few days ago because they want to build a professional-looking site for a friend, and they're comfortable with HTML and CSS, but they don't understand how Wordpress works.
I did find a few things that I think needs some work though.
You're presenting a commandline interface. I love commandline. I much prefer a sensible CLI to even the best graphical interfaces. But your target market? I'm not sure they'll like that nearly as much. If they were comfortable with CLI, they'd be probably be comfortable enough to find their way around a server. Is there any way you can manage testing & deployment without the CLI?
On your site, you advertise the virtues of a static site as being fast. But, in the video, images are loading slowly on your demo page. It really stood out to me. You might consider having the page's images already in your browser cache when you load the demo page.
And the video -- the video is really a bit rough. You're clicking around way too fast in the video. I had trouble keeping up with what you were doing. The page template you're using in the video isn't sexy at all, so that makes it less attractive to designers. Having some kind of default page template along the lines of what you'd find for Wordpress would probably be good.
The video was also too specific. You went into some detail on embedding podcasts or Soundcloud stuff, but didn't really explain any of the rest of the interface. In your intro video, I don't necessarily need to know step-by-step how to embed a podcast, I maybe only need to know that it's possible. At the end of the video, I still felt like I had no idea what Webhook could do, which sucks because although I'm not your target market, I probably know 6 people who are. Along the same lines, there were a number of typos and miscapitalized things in the video, and some of the dialog was a bit hokey (like when you remembered that you actually had to go into chat to invite a participant). This adds to the "rushed" feeling in the video.
And the thing is, it looks like you've got a really polished product! Then you have the video, which makes it feel a lot less professional.
Also, it seems like you could tap a secondary market, by hosting a template shop on your site and taking a cut from sales of the templates. Just because someone's comfortable with HTML & CSS doesn't necessarily mean that they want to start from scratch. With the explosion in popularity of things like Bootstrap, I think there's pretty good evidence that people are avoiding that as much as possible.