6.6, and section styles that an editor understood

Style variations applied to a block and its children, which is the thing editors had been asking for by describing it badly.

{
  "version": 3,
  "title": "Inverted section",
  "slug": "inverted",
  "blockTypes": ["core/group"],
  "styles": {
    "color": {
      "background": "var(--wp--preset--color--ink)",
      "text": "var(--wp--preset--color--paper)"
    },
    "blocks": {
      "core/button": { "color": { "background": "var(--wp--preset--color--paper)" } }
    }
  }
}

The nested blocks key is what makes this a section rather than a colour swap — the button inside an inverted group needs to invert too, and every previous approach to that involved a class and a stylesheet. Editors had been asking for “a dark bit”, which is exactly this and was impossible to give them without a developer.