Thank you. The architecture to get an HTML/CSS input layer in front of VMPrint is already there via a component called Transmuter. I have made transmuters to allow markdown with custom syntax support as input, and they work extremely well. Full support of HTML/CSS will be still be difficult, but a well-defined subset should be easy. The sibling Layoutmaster project also demonstrates this -- you can hand it a DOM element and it automatically grabs the styles and contents, converts them into the JSON AST, then feeds them to the engine for instant layout.

HTML/CSS is notoriously bad for print.

Basic text styles are ok, but things like authored pagination, page header/footer, mirrored margins, margin notes, footnotes and references are basically unsupported or need to be hacked together.