HTML and CSS Best Practices
Key to success: Modularity
HTML and CSS deserve to be as DRY as your Ruby is. This is difficult to achieve in raw HTML and CSS.
HTML - Templating Libraries
Ideally - especially for large sites - HTML is used with a templating library (erb, handlebars, jade). This allows you to access to variables, ability to loop over elements, and eliminate most needs for hard-coding/repeated code.