I haven't found much info on this, perhaps because it's not that complicated. Is it as simple as selecting low wattage components in a decently ventilated case and calling it a day? Maybe an APU instead of a CPU + basic GPU? My parents could use a new machine soon and I volunteered to put build one for them. It's "just" a general use desktop machine, the fanciest thing it'll have to do is play HD video in like the 10th tab open in Chrome. Their old computer is pretty old and struggles with the basics these days.

It'll be in a semi-enclosed area so I'm thinking a smaller form factor might be good as well to give it a little extra breathing space. But the primary goal will be to generate less heat, rather than just manage heat better. mITX might be cool, but I'll have to check if they currently or may have use for more than one PCIE slot.