Technical accessibility
Also known as: technical health, fetchability
Whether a machine can get a page cleanly, quickly, and on the first try. One of the four readings behind a visibility temperature.
The physical read: HTTPS, redirect hops, server response time, document weight, viewport, language and character set declarations, a reachable robots.txt, and alt text coverage on images.
These are the cheapest points on the scale to earn and the most irritating to lose, because almost every one of them is a configuration decision rather than a piece of work.
Response time here is time to the document, not time to a finished render. A slow number is a server or a redirect problem, not a front-end one, and optimising images will not touch it.
Alt text sits in this reading rather than in content because it is the only description of an image a text retrieval system ever sees. It is also the same work as the accessibility requirement you already have, which makes it the rare signal with two unrelated reasons to do it.