Phase 2 of the consensus banana is to move classes out of core templates and into a "classy" base theme which Bartik and Seven will extend from. This will change the markup (classes) of any Drupal 8 themes that do not have a base theme specified.
Add classy.info.yml to core and make an announcement so that contrib themes/base themes that currently do not have a base theme specified can immediately change their THEMENAME.info.yml to specify classy as their base theme. This will minimize the disruption when the currently-default core classes are later removed from the core templates.
User interface changes
Themes with no base theme specified will have some class/markup changes later.
PASSED: [[SimpleTest]]: [PHP 5.4 MySQL] 78,673 pass(es). View
FAILED: [[SimpleTest]]: [PHP 5.4 MySQL] 76,186 pass(es), 15 fail(s), and 6 exception(s). View