Westoncb said it quite elegantly: "I think the core idea involved is a shift away from using text as a model for representing programs; instead, interact with more abstract representations of code, and render those abstractions as text. This allows your editor to have better understanding of the language you're using, so syntax becomes a property of a language's visualization rather than something totally central to it (and now you don't have to memorize it!)."
Foolproof HTML of course tries to do that only for HTML, but I do hope I can expand it to CSS, JSON and maybe generalize it later on.
So I'm not advocating for any specific syntax, but rather a way to edit html so you don't need to care about the syntax. The HTML syntax would still be there in the .html (or .erb, .php or whatever your template language is) the same as it has always been, but you could visualize it in a way that's most comfortable to you and edit it as a structure, so it would be impossible to produce syntax errors.
This could end up being a text editor plugin or a stand alone app, some sort of library or kind of anything, but I'm prototyping interactions and ideas as a standalone web app because that's what I'm good at.