- **Docs code blocks in `mermaid` or `chart` can show a preview.** A `mermaid` block shows its diagram (Mermaid 12, bundled and loaded on first use, drawn in a hidden frame whose policy allows no requests, text labels only; sources that name an image, icon or URL are also refused, and the SVG is sanitized and isolated, so links, scripts, HTML labels and external loads are removed; at most 5 000 characters and 150 connections, and slow diagrams or a page whose previews took over a second wait for Preview). A `chart` block reads a strict JSON spec and draws a bar or line chart in the product's series colours, with a summary for screen readers and a "Show data table" alternative. Errors show in that block above its source; the document never changes. Comments anchor to the block's source text, not to diagram nodes or chart bars.