Demo
This is what a rendered plan looks like:


The viewer follows your OS appearance and uses the Default colour theme until you choose otherwise in Settings; appearance and colour theme are separate controls.
See the full rendered plan in your browser.
Some parts of this plan are ordinary Markdown.
But take a look at the code diff and the warning under Risks and mitigations.
Those use CodeDiff and Callout, two of the typed components Big Plan ships for presenting common plan information in a first-class way.
The diff adds line numbers, counts, change coloring, and a line-anchored annotation without asking the agent to design custom HTML.
The output runs locally without a third-party service. The two-artifact delivery ADR owns the delivery and script-behavior contract. To see it in action, install it.