We should divide wiki's style into well defined tiers such that every rule is motivated, well placed, and available before elements are rendered.
We suggest five tiers, three of which are new or rarely used in our current css strategy.
Tier 1. Broad rules that define the overall look of page and plugins should be included in the static html so that initial rendering is fast and stable. (new)
Tier 2. Style details for user interactions ranging from hover to editing should be present and well organized in the single static style.css.
Tier 3. Branding such as background should cascade from an optional, site specific style sheet. (rare)
Tier 4. Plugin specific and appropriately qualified styles should be one-time loaded with a plugin. (new)
Tier 5. Plugin specific inline styling of individual elements as information coding not covered in Tier 2 is acceptable.
# Resources
Mozilla suggests coding techniques. post 
W3c grid layout. post
A11y diagnostic bookmarklet. site
CSS Purge, saving the web. site
Debugging CSS. post