Hello there!
First of all the video-wall solution is simple genius!
Is there a way to have a html-wall?
You rule!
Not right now. All the browser content at the moment is handled quite awkwardly. Additionally once you have dynamic content, things get a lot more complicated: You’d certainly don’t want a wall where (for example) each corner shows a different version of a website due to each of them retrieved at a different time. So you need additional communication between the Pis to exchange a single view of the page. It’s totally doable and there exists non-public code that does something similar for non-browser content, but getting that right is really something.
Anything dynamic on the page itself (think animations and stuff) make that pretty much impossible as there’s no real way to sync that across devices.
Bummer, but I totally get why it’s so complicated, meanwhile I will just work with video when the content is for video-wall’s 
Thank you so much for your quick response!