What are the risks of storing E10 gasoline in a gallon HDPE container for a few months, vs storing in a proper plastic fuel can? Assuming the HDPE gallon is thick (used for Hydrochloric acid before, and was properly rinsed and dried). I'm interested in knowing what are the technical differences that make the legal plastic can so much different that a regular container, since they are both made of the same material with similar thickness.
I wouldn't recommend it because the water content in the gasoline can react unfavorably with residual acid and/or the plastic is not meant to be in contact with a chemical like gasoline. You should only transport and store flammable liquids in specific containers meant for that purpose.