Liquid mode? It looks like a tool to reflow the content in existing PDFs, which are often intentionally not responsive. I don't see how this is similar to HTML.
PDFs can do all sorts of voodoo (like you can do with HTML if you hate the user's browser) to make legible content that is pretty illegible to machines - but most documents are produced by tools that have pretty sane outputs that can be reverse parsed to get a pretty nice HTML blob.