Almost Free Text is a document preparation system. It is mostly free form meaning that there is little intrusive markup. AFT source documents look a lot like plain old ASCII text.
AFT has a few rules for structuring your document and these rules have more to do with formatting your text rather than embedding commands.
Right now, AFT produces pretty good (weblint-able) HTML, XHTML, LaTeX, lout and RTF. It can, in fact, be coerced into producing all types of output (e.g. roll-your-own XML). All that needs to be done is to edit a rule file. You can even customize your own HTML rule files for specialized output.