I finished the article and was wondering if the author will show a image of how the dot looks like, and later find out that the first "image" is a video, and Firefox blocked the auto-play leaving the first frame that doesn't contain the dot. Have to right click on it and select play to reproduce it. I like that feature and avoid things moving around without interaction, but I think Firefox should put an overlay with a play button on native web videos, at least on mouse hover, isn't the first time I have to right click to know if is a video.

And for the author, the use of descriptions at the bottom of images/videos is good for accessibility, a description like "Screen recording of the dot appearing" will avoided my confusion, and persons using screen readers will know it too (I checked and in the code there isn't a alt text for that (question: the alt="description" works for <video> like is used in <image>?)).