Danny Van Kooten wrote an great article. He wrote how he tries to reduce the environmental impact of his WordPress plugins:

Just last week I reduced global emissions by an estimated 340.000 kg per month by removing a 20 kB JavaScript dependency in Mailchimp for WordPress. There’s no way I can have that kind of effect in other areas of my life.

But he also gives tips on how everyone with a website can decrease the impact of the own website:

Your content site probably doesn’t need JavaScript. You probably don’t need a CSS framework. You probably don’t need a custom font. Use responsive images. Extend your HTTP cache lifetimes. Use a static site generator or wp2static.com instead of dynamically generating each page on the fly, despite never changing. Consider ditching that third-party analytics service that you never look at anyway, especially if they also happen to sell ads. Run your website through websitecarbon.com. Choose a green web host.