The tricky and interesting bit is figuring out actual emissions (vs applying a rule that 1 visit == x grams of CO2). Work in progress:
we've committed to making our search engine site at breezethat.com as minimal as possible -- hand-coded HTML, etc. and even now it's still too big -- one big next step is creating a boostrap & fontawesome subset package which should reduce our total asset size by 50% or more
we haven't and probably won't minify for a while mostly cuz wholly bootstrapped and it's one layer of potential debugging for which we don't have spare bandwidth
we'll likely switch to images off by default for similar reasons + speed
the most striking thing is when going to regular sites and running into all the bloat; working on a separate extension people can use to mitigate that and also better set options-per-site in general.