Ask HN: Can the 80/20 rule be applied to learn programming?

I'm just wondering. I've been doing classes on CodeSchool and some other free resources. After reading a lot about how 80/20 Rule can be applied to other areas of my life, I started to wonder if it applied to software engineering too. I understand that syntax is obviously a prereq, also that 20% of your code will be what users need 80% of the time, 20% of your users produce 80% of the content, etc. What I'm specifically wondering is how to master Ruby or Python for backend and HTML5/CSS3/JavaScript Libraries for frontend, with 80/20 principles. Thanks for any input or resources.