There's so much text but nothing that clearly says what it is.

Is it HTML and its contents (e.g. images) in a binary format?

yes exactly.

you can go devtools at https://hmml.eddocu.com

it downloads single binary that contains media assets (svg, image, video, ..) and html/css blueprint, even js (security concerns!)

Common side-effect of letting Claude write your landing page.

Apparently it's mostly this:

    // ENCODE — pack(html) → one .hmml (a Uint8Array you store or send)
    //   · lifts every data: image out of the HTML into raw bytes (no base64)
    //   · gzips the HTML/CSS/JS and frames it all as one binary blob