In reply to Tro.

Also to add on to this, this customizer communicates via postmessage from the parent window to iframe. This keeps the separation of controls and related styles separated from the sites styles - instead of the div soup and overly specific style required to not clash with editor controls. Not to mention the defacto example WordPress set of using body class for managing complex specificity that the classic editor was capable of doing as it was loaded in an iframe. This is still the biggest issue with Gutenberg imo. They need to either utilize shadow Dom, which they won't because of legacy browsers, or communicate with an iframe. Themes in theory should be able to simply enqueue their frontend styles into the editor without writing a massive amount of code to just get something halfway decent looking. Gutenberg does a lot of great things though, but the customizer is what people know and use for "global styles". I have gotten to the point where I create a global styles plugin for Gutenberg and offer per page/post overriding or full site overriding. Then duplicate the same full site overriding in the customizer. If Gutenberg continues on to fill site editing, the primary focus should be the customizer. Replace it's API with Gutenberg's similar to how wp.editor's functionality still mostly worked in tinymce and Gutenberg. If there was parity between posts/pages and the customizer, then things would really start to come together for users. Having two separate design systems makes it confusing for most novice users. If full site editing is the future goal, Gutenberg should move forward with it, and not try to reinvent small components in an interface where it doesn't entirely make sense. Just think if you were to go into the customizer or any page/post link took you into it, you could easily have control to change headers/footers/templates, and update those things with block content if the Gutenberg system was already present. Why not just click on text in your full site preview, drag block in/out of your editor into different drop zones? It feels like people want this, but are putting too much thought into it. It can already be done today with minimal code, and still retain backwards compatibility. Maybe it's just too many cooks in the kitchen, but Gutenberg was announced forever ago, and took a long time to get to where it's at today, which arguably isn't really anywhere that is useful for most theme developers. As a side note I really hope that theme authors can create blocks not as block plugins to include in their themes. Block styles can get most of the way there, but not being able to add additional controls to something like a simple blockquote to change backgrounds and or text styles is very limiting. Don't get me wrong you can go far with block styles (I have made over 100 different ones at this point), but sometimes you just need to insert an extra element, or provide an extra control which makes the user experience even better.