Open the diagram full screen, then mark it up: draw with the markers, point with the laser or a lightsaber, leave Post-its, and export what you’ve made. Paste in your own Mermaid diagram to draw on that instead. It all stays in your browser.
Alice, Bob and John
A sequence diagram: Alice greets Bob, Bob asks John and answers Alice, and Bob, after a long think, checks with John, whom Alice then greets too.