As someone pursuing my own vision of a hypercard-like multimedia environment that lives in a self-contained HTML document, I'm quite interested in learning abou

As someone pursuing my own vision of a hypercard-like multimedia environment that lives in a self-contained HTML document, I'm quite interested in learning about the approach mmm.page takes with interactivity, but attempting to add "custom code" just tells me I need to upgrade. Is there somewhere with more details?