I think it seems odd to you because it is a new concept and you have not yet read about it.
This is an HTML editor that is built-in in most browsers. The main difference is that it supports previewing template marks. This means that if the HTML has for instance a mark {navigation} it automatically shows a preview of the navigation bar.
The advantage of this approach is that you do not need to save the HTML template to see how it will look like in the site pages.
The template marks appear with a dashed border so you can see to what each template mark was expanded.
Some marks support alternative layouts. Those are the ones that appear with the + sign in front of the template variable name in the preview, like for instance {navigation} + .
If you click inside the dashed area of those marks with the + sign, it shows a pull-down menu with alternative layout options.
If you click on one any of those options, you can switch the layout. For instance, you can turn the navigation bar layout to make it show vertically. The layout changes are previewed immediately.
Anyway, this was all explained before several times but I suppose you did not see it. That could explain why you found it odd.
There was even a screencast video showing step by step how to use all aspects of the design proposal system, including the HTML editor.
phpclasses.org/blog/post/110-Screen
...
phpclasses.org/blog/post/104-Submit
...