Drupal 8 HTML markup and semantics
Why should it change (drastically) -- it embraces a base of HTML5 markup; anything beyond that essentially becomes business need, accessibility requirements or simply put designer preference on what clean and simple means to them.


Mobile Initiative
Design Initiaive
HTML5 Initiative


From HTML5 Initiative page


  We want to ensure we're spending our time implementing features that will directly benefit Drupal's user base the most. As part of this initiative we'll focus mostly on:
  
  
  Adding support for the new form elements to Drupal's Form API.
  Adding new semantic elements in core templates in an appropriate way.
  Adding ARIA roles to markup to improve accessibility.
  Simplifying style and script elements.
  Ensuring input filters and functions accept HTML5
  elements.
  




Does the above goals mean that Drupal 8 HTML markup will be as lean turning on Fences in Drupal 7 and nuking all built-in markup? No, will the base markup be too complex, hardly. It will be designed to meet most peoples needs.