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Drupal has new CSS coding standards. http://drupal.org/node/1886770
Zen should follow them.
We're pretty close already, but we need to go through our Sass files and tweak.
How do you deal with mixins/extends? I think most of those fall into the "important property to understand the ruleset". Which means they should appear first. @extend first (since the extended ruleset is most likely before the current ruleset) and then @include mixins.
Our generated CSS may need to slightly break Drupal's coding standards since we have no control over what order mixins add declarations to a ruleset.