Social Links profile using HTML & CSS

What are you most proud of, and what would you do differently next time?

Was not too difficult but got stuck at the very beginning with a stupid syntax error.

What challenges did you encounter, and how did you overcome them?

Put a ',' in my margin property and could not see the difference from my earlier uses. Had to post it up for someone else to show me...so I'm hoping I will be less of a dumbass from now on :/

What specific areas of your project would you like help with?

As usual just wondering if I'm actually using best practices. Should I be hard-coding width and height on these elements or using some other more responsive type of units or percentages or something?