By comparison, average UK cars put out 124.5g for every km driven.

It's not nothing, but it's not a lot either. It probably is worth evaluating the electrical efficiency of the tech industry as a whole, both in terms of highly executed code hotspots and entire pieces of business of questionable utility (ad javascript, cryptocurrency proof-of-waste), but individual web pages are the plastic straws of the industry compared to the giant flare stacks of real problems.