But that's not done for human readability, that's done for machine parsing? A human would understand just as well:

  props: mime_type: "text/html", encoding: "gzip" # Inline dict.

My mind has a feeling that this might be possible interpretation:

    props: [mime: [html, encoding:[gzip]]]

There are many different humans. I definitely like the idea to separate “:: vs :”.

For me, it’s clearer with a double colon. Not intuitive, but extremely easy to get used to. When I see the first colon introduce a list, I have to go out of my way to not see the other colons as introducing lists.