Matthew Butterick, in his online book Butterick's Practical Typography:
This is a bold claim, but I stand behind it: If you learn and follow these five typography rules, you will be a better typographer than 95% of professional writers and 70% of professional designers. (The rest of this book will raise you to the 99th percentile in both categories.)
All it takes is ten minutes—five minutes to read these rules once, then five minutes to read them again.
A short, concise ruleset that can make any website look more professional. So often I come across blogs that would be 10x more readable if they accounted for rules #3 and #4 alone.
If you end up reading through Mr. Butterick's typography guide in its entirety, consider paying him a few bucks (or more) in thanks. He did a wonderful job with it. (Tip: hyperlinks throughout the guide are indicated by prepended diamond symbols.)