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Render elements used in a form can be nested. If nested child element have errors these errors are internally recorded on the $element['#children_errors'] property. As of now the count of these children errors are exposed to the front end as the DOM element data attribute data-child-error-count. In addition these elements get the class form-wrapper--child-error.

Furthermore, details elements, (collapsible) wrappers, get the class details--child-error. For accessibility a non-visible text is added to indicate the error state.

Vertical tabs are details enhanced by JavaScript to allow tabbing. The JavaScript theme function used to render this element (Drupal.theme.verticalTab), now receives the error count as well via the provided settings (settings.childErrorCount).

The following shows what has been done with this in the theme Seven (highlighted inactive tab):

Vertical tabs errors

Impacts: 
Module developers
Themers
Updates Done (doc team, etc.)
Online documentation: 
Not done
Theming guide: 
Not done
Module developer documentation: 
Not done
Examples project: 
Not done
Coder Review: 
Not done
Coder Upgrade: 
Not done
Other: 
Other updates done