Drupal Core markup in continuous upgrade path

Drupal 8 introduced the concept of base themes in core and shipped with two new base themes; Stable and Classy. These themes were both created with the prerequisite that they could be changed between major releases. This might have made sense at the time since until then, upgrading to a new major release was a heavy task. However, since then Drupal Core has adopted a continuous upgrade path meaning that people should be able to upgrade to the next release without having to make any changes, as long as their site didn't use any deprecated code. Since introducing deprecation workflow for HTML and CSS is impractical, we will have to create separate policies and workflows.

This presentation will cover:

• Why introducing deprecation workflow for HTML and CSS is impractical
• Insights from maintaining our HTML and CSS and why we've been unable to make any meaningful progress since Drupal 8.0.0 on modernizing Drupal Core default markup 
• What changes have been made prior to Drupal 9 to make Drupal 8 themes continue working with Drupal 9 
• Some proposed changes to enable us to make improvements in the future to Drupal Core default HTML and CSS