What is old is new again....
I remember back in the 70's my father had a steel, rectangular 2 gallon "Eagle" gas can and a smaller 1 gallon can for the weed trimmer. They looked just like the 1 gallon cans that paint thinner, etc. comes in. They had a plastic spout with a snap-on cap and the spout was internally vented with a little tube molded into it up the inside. They were excellent to handle and didn't drool, drip, or spit (reminds me of an ex-girlfriend....)
I have some generic 5 gallon plastic cans now and I put on some of the "push" spouts that stop the flow when you pull them off the fill neck of the tank. They work fine, but I leave the vent slightly open all the time as one day it started out cool and the can was about 1/4 full. I opened the garage (faces west) to get the tractor out and went about my mowing. The can was sitting right in the hot sun and I swear it looked like a tick about to pop.
I slowly opened up the vent and it really hissed. I don't know what it would take to pop one of these cans, but a sealed plastic can full of gas sitting in the sun on a 90 degree day is sure going to test it out.
The gas vapors are going to get into the air one way or the other. Sealing up the can is like putting a big cork in your a$$. It's gotta come out at some point. CARB is a useless waste.