It would be a fantastic feature to be able to collapse elements based on headers or nested bullet items.
I know when editing large markdown files that I wish I had the same abilities as in an IDE of being able to collapse particular sections.
Alright, I consider to implement it. Thanks.
It is already there! It has 2 minor issues:

So try this:
<details>
<summary>whoa</summary>
* blah
* blah
</details>
this is kinda broken in v0.11.3 if there is a newline in the details.
@abbaspour Yes, it is broken for now since we introduce sanitization. If you want to use it, you have to disable sanitization.
@vangourd I'm going to close this issue. If you think pihentagy's answer isn't enough, let me know! 馃槃