PostCSS usage is broken in editor if Yoast is used.

[tcmulder]

When Yoast is active, the editor canvas iframe falls back to the old div.editor-styles-wrapper version that attempts to scope all CSS. When using PostCSS, currently nested CSS gets output as nested CSS, since it has support in all modern major browsers; but, in the old div.editor-styles-wrapper version of the editor, this causes parent CSS to have .editor-styles-wrapper nested between them and their children.

Issue opened here for that issue.

Solutions:

1. I'm going to try just using scss for now, as it seems to perform all the nesting translations back to an older version of CSS.
2. Maybe I could get PostCSS to disable nesting. Wasn't finding very clear documentation on how to opt out of common CSS functionality.
3. I can enqueue the CSS in the enqueue_block_editor_assets hook, but things break outside the editor: nothing is scoped in the old div.editor-styles-wrapper version of the editor.

[tcmulder]

Here's the ticket in Yoast: Yoast/wordpress-seo#20535