Drupal 8 is still beta-ish in so many aspects, that “workarounds” is becoming a separate genre of Drupal development tips and tricks.
The today’s workaround is for the bug in “node preview” functionality. It is not frequently used, but can be rather convenient for large websites with moderation enabled. The bug can be spotted only when CSS styles, applied to the node/page template rely on the <body> CSS classes, namely the core-added page-node-type-[node bundle] body class. It helps to effectively distinguish different node templates, and therefore are widely relied on by front-end developers. However, in preview mode, this class appears entirely missing, which breaks the styling.