A <content> element: static text, an expr, markup, or (representably, if not validly) any combination.

Spec 5.6 allows <content> children to be "text, XML from any namespace, or a mixture of both". Preserving non-text markup would mean holding a DOM subtree inside a Document struct, crossing the layer boundary this rewrite is organized around - so markup holds a verbatim source slice instead (ADR-0041), not a subtree: no DOM node ever reaches this struct. The slice is opaque bytes here; the child document it describes is only compiled at invoke time, from markup, through Statifier.Invoke.Source.resolve/2.

markup is nil unless <content> has at least one element child; when present, it is the exact source bytes from the start of the first non-whitespace child to the end of the last non-whitespace child, elements and text runs alike, so a 5.6.2 "mixture" slices whole. Unlike text below, this slice does not fold line breaks - it keeps its raw CRLFs and lone CRs exactly as written; the parser's XML 1.0 2.11 fold only applies when the child document markup describes is itself parsed, at invoke time (ADR-0045 decision item 5). markup_location is that slice's own span in the parent source - its start_offset is what makes a byte offset inside the child document translatable back into a parent-document offset by plain arithmetic (ADR-0014).

text is Statifier.Parser.DOM.text/1's concatenation of the direct text children, folded per the parser's XML 1.0 2.11 line-break fold (ADR-0045), and has no span of its own: an entity reference or a CDATA section can split that concatenation across more than one run in the source, so there is no single span to give it. This Content node's own location is what a diagnostic about the text points at instead.

expr, text, and markup are all nilable and all representable at the same time, even though spec 5.6 says a document MUST NOT specify both expr and content. That mutual exclusion is a validator concern, not this layer's - holding all three is exactly what lets Statifier.Validator.Checks.Content report a document that violates it, as {:content_expr_and_text, expr} at this node's own location. Whitespace-only text is source formatting rather than a payload and is not reported.