Excerpt below:

"Readers of a certain age may remember early e-books that had black rectangles in place of the images. (They were rendered as such because publishers had only obtained print rights to the images.) That’s how blind people experience many books—and other publications and websites—to this day, missing what the images convey to sighted people.

I’ve written often about how much easier it has become to make books accessible these days. That’s because a properly tagged and structured EPUB can be accessible without further remediation—i.e., “born accessible,” the same EPUB everybody gets.

Except it’s a bit more complicated if the EPUB files have images. Images need what is known as “alt text”: image descriptions that convey to a blind or other visually disabled reader through text what the image conveys visually to a sighted reader. Those are overlooked in most books (with the exception of recently published books for higher education, where the requirement for accessibility has become an imperative)."