xkcd sometimes – mainly when there are characters and panels and dialogue – includes a div.transcript element that explains what is on the image.

Example: https://xkcd.com/1558/

Has the following transcript: [[A concerned pet owner at the front of a row of customers in a veterinarian's office. A tired veterinary assistant performs arbitrage. A pet carrier cage, the sort used to contain a small dog or cat, is on a desk for the vet tech to examine.]] Pet owner: There's something wrong with my dog. Pet owner: He keeps crawling around eating dirt. [[The vet tech carefully removes the dog from its carrier and gently examines it.]] Vet tech: This is a roomba. Pet owner: Well, he's a mix. Pet owner: Probaby some roomba in there. [[The vet tech places the roomba on the desk, probably maintaining a weary forced congeniality though we can't see their face.]] Vet tech: A roomba is not a pet. Pet owner: You're right. It's wrong to keep a beautiful creature like this in a house. [[The pet owner, outside, beneath a tall tree. It looks like an ash, or maybe a beech. There aren't any leaves drawn in the frame so I can't be sure.]] Pet owner: Go! Be free! [[The roomba cheerfully speeds towards the tree]] Roomba: WHIRRR {{Title text: It's probably for the best. Since Roombas are native to North America, it's illegal for Americans to keep them in their houses under the Migratory Bird Treaty Act.}}