Clean CSS HTML Code for Omega 4

Hmm... Any specifics? We highly optimized the output and CSS already. Probably more than any other theme to date :P (although mothership might be a bit more drastic when it comes to reducing the amount of classes and html wrappers).

The thing that made me cringe when I first came to Drupal was the sheer amount of divs everything is massively subnested inside. This also pisses off any designer who comes to look at a Drupal site :p

I'm not sure how much is easily controlled by the theme though. I feel like Drupal lets you add wrappers in most places, so it doesn't seem like many should really be needed out of the box. Glad to hear you're cutting down in 4 :D

I am currently working on an extension that allows more fine grained control on things like field output. However, we can't do anything more aggressive than what we have now (which is already quite optimized compared to D7 core) in as default because it is going to break a few modules' html output / styling.

maybe using mothership as a base theme of the omega base theme, like mentioned here:

https://drupal.org/node/1503286#comment-5786416

this make sense?

Nah, that's not really an option. Both, mothership and omega have their own code to clean up HTML/CSS as much as possible. Also, at this point they have totally different approaches as Omega 4.x has adopted the D8 CSS standards already (and also for most of the core CSS/HTML) so there's not much point. It's very very clean as it is already. Anything beyond this would be too intrusive and cause problems with many existing sites and be very frustrating for ppl working with Omega.

Great!
Thanks for the clarification!