Proof of concept

Markdown → remark/rehype → Mermaid rendered to inline SVG at build time (headless Chromium). The browser never downloads Mermaid.

Diagrams exist in the static HTML → SEO-visible, no layout shift.
Theme switching via CSS custom properties baked into the SVG at build time → one diagram, both themes, no duplication, no re-render.
Source view is a native <details> element → zero JS.
Only client script on the whole site: the ~10-line theme toggle.