Here are a few interesting CSS links garnered from around the web over the last couple of weeks.
More flexbox advice from CSS Tricks
Guide to using px, ems and rem from CSS Tricks again.
Stripes in CSS
Responsive tables with complex data
If you struggle with specificity then this specificity calculator may help you.
Removing unused CSS
Parrallax scrolling with CSS.
More to responsive design than you think.
Do responsive designs look the same.
Responsive email template from Email on Acid site.
12 little known CSS facts from Louis.
You can also view a list of previous posts here.