[Zuiii]

You're right.

We could have such a format if browser and os vendors were interested in supporting such a use case. Unfortunately, they aren't.

On the browser side, supporting all-in-one html files can be as simple a reading a single multipart-encoded page. Heck, if they support automatically serializing all external resources as datauris when saving pages, then most browsers will be able to open them without any modification.

On the OS side, operating systems can treat html files as first class citizens; execute them in an offline sandbox (most operating systems have embedded webviews), then extract icon, title, description and other metadata to present to the user. An icon the consists of a blank page with a small browser icon in the corner doesn't tell me anything about what the page is about. This needs to change.

In short, html can be easily made nicer to deal with locally thanks to all the parts already being in place. The problem is that no one (tech giants, os vendors) are interested in doing this.

[Flimm]

.mhtml (or .mhtm) is that format. It's an archive containing an HTML file along with all the resources it references (JavaScript, CSS, and images). These browsers support it: Internet Explorer, Edge, Opera, Chrome, Yandex, and Vivaldi. Create one by saving the web page and choosing the .mhtml format. Safari supports another format called webarchive.

https://en.wikipedia.org/wiki/MHTML

[GoblinSlayer]

There's epub as one file html document.