Typesetting systems, such as LaTeX, permit users to define custom markup and corresponding formatting to simplify authoring, ensure the consistent presentation of domain-specific recurring elements and, potentially, enable further processing, such as the generation of an index of such elements. In XML-based and similar systems, the separation of content and form is also reflected in the processing pipeline: while document authors can define custom markup, they cannot define its semantics. This could be said to be intentional to ensure structural integrity of documents, but at the same time it limits the expressivity of markup. The latter is particularly true for so-called lightweight markup languages like Markdown, which only define very limited sets of generic elements. This vision paper sketches an approach for user-defined semantic markup that could permit authors to define the semantics of elements by formally describing the relations between its constituent parts and to other elements, and to define a formatting intent that would ensure that a default presentation is always available.