The styling of the dialog element needs adjusting.

Awesome initiative, I've been impatiently waiting for full support on the html element - thanks for taking it even a step further!

As to the styling, avoid overdoing. The required additions should rather go into the core themes. Module-related CSS will just make it harder to override in custom themes whereas native theming support in core should help a lot. After all, this is just an html element ;-)

@hexabinaer thanks, yes that's correct! We should eventually try to replicate the previous classes and styling in the adapter CSS as far as it makes sense. For the native css we should do as little as possible / required. Eventually just fixes to the HTML element, if needed.

Rest should go into core templates.

I think we need here:

`dialog.dialog-native:not(:modal) {`

1. avoid usage of html `dialog`. It is not really good in BEM point of view. Instead it we need simple class `.drupal-dialog` with BEM style elements. EG: `.drupal-dialog__header`

2. Avoid complexity in CSS https://web.dev/articles/reduce-the-scope-and-complexity-of-style-calcul...
Everything should be styled with lowest possible CSS complexity. EG: `.drupal-dialog__header` :)
It was huge problem with jQuery UI `.ui-dialog .ui-dialog-header ... {} `

3. Use css custom props everywhere. It will give lot of theming options for developers.

@thomas.frobieter are you still working on it?
I have some ideas.

@finnsky Not started yet, and I can't tell when this will happen, as I am currently very busy. So, feel free to add your thoughts here!

I think @thomas.frobieter is quite busy currently, but a quick review shouldn't be a problem, so feel free to work on it yourself @finnsky! :)