Docbook

XML is vastly different than HTML, but the concept is very similar. If you are good with HTML, then XML will feel like the “pro” version of what you already know.

XML is rigid in what its processors accept, so there is an absolutism to how you structure your documents. This may help you organise information better, and it guarantees predictable output in the end. You won't spend time shifting indents and special meta-characters around in your text editor; you will spend time writing content within a well-structured framework.

XML is a long-standing format, and Docbook is a well-respected schema. Docbook is well documented on http://docbook.org and XML is so well-known that you can take classes on the subject.

One your text is in XML format, it is structured and predictable. This probably means that if there is another format (html, epub, pdf, ps, plain text, rtf, odt, and so on) that you want to output to, you can convert to it from XML. There just isn't any ambiguity about XML, and heaps of post-processors.

The process of creating well-formed XML is not simple. It is a very verbose format, it will fail at the smallest error, it enforces inheritance, and it requires some number of post-processors in order to get it out of the XML format.

Unlike markdown or HTML, XML is intolerant of any deviation from its defined schema. Something as simple as a missing closing tag will break the processor. There are tools, such as xmllint to help ensure well-formed XML, but it is not uncommon to attempt at least three builds before a successful one.

The look of documents output from Docbook are clean and professional, but to change the look and feel of your output, you probably need to learn XSL. XSL can be complex, especially if you have only just learnt XML and how to process it.