I once basically did just that, besides the image dithering. I ran a small blog off a Raspberry Pi, only using Markdown files and something to render them to HTML. I used my own IP address, which I know you're not technically supposed to do but in 15 years I've never had an ISP crack down on me for it, and had a script that would run periodically to update the A record for my domain in Route 53. (since my IP was dynamic)
Whether you're doing it for energy purposes or not, it's a fun project because it feels a bit like sticking it to the man, since we're used to always hosting our sites on someone else's machine.
My guess is that this is mostly a way for them to object if you start using too much bandwidth. Most ISP's "guarantee" way more bandwidth than they can actually provide on a daily basis. If you're hosting a website that gets low traffic, normally, they probably will not only not know, but not care. Just my guess.
For several years starting in 2010 I ran my personal website off a Pentium III Thinkpad running Plan 9 sitting on the floor behind my desk. I wrote the web server in Go; it was a "hybrid" in that it served mostly static files, but in the blog subdirectory it would render Markdown to HTML.
It worked great! I eventually migrated to a VPS when the hardware started to get too flaky with age (and years of accumulated dust sucked through the fan inlet)
Whether you're doing it for energy purposes or not, it's a fun project because it feels a bit like sticking it to the man, since we're used to always hosting our sites on someone else's machine.