I'm a little surprised that both SILE and Patoline [1] both seem to stick with the TeX formatting tradition of \begin{foo} ... \end{foo}. Is this really the best way to express layout? Or is this so that users will feel comfortable with new systems? I would have thought that taking a cue from some of the markup systems like Markdown, ReStructuredText, ASCIIDoc, etc. might have been a consideration. Just curious about this design choice.
One markup language is much the same as another. The real issue here is the fact that typography is not inherently easy to specify in a serial language. It's an intuitive, visual activity, for which direct manipulation and WYSIWYG are ideal. Any non-trivial layout contains both typographic hierarchy and spatial hierarchy, grid systems and the like.
Knuth blazed an important trail but you couldn't typeset a magazine in TeX. It's all very well to scratch one's own itch, of course. It is just that the specfication of layouts is a more complex, intuitive and holistic problem than TeX-like languages acknowledge. You need to be able to work with intrinsically visual and spatial things like clip paths, not to mention checking optically for rivers etc.
It seems to be common for a certain kind of programmer to be detail-oriented when it comes to coding but tone deaf to the richness of typography as craft. Grrr.
[1] http://patoline.org/