These are meant to be injected via Stylus, an extension that allows customization and styling of various websites. (It's likely these styles would work in other tools allowing for customization or adding CSS, but I haven't checked any other tools personally. This uses pure vanilla CSS.)
This tool is currently designed to:
- highlight images with no alt-text in red
- show you the alt-text for images that have it when you hover over the image
- [requires extra configuration] hides images from your timeline (note: this won't hide them if you actually click the link)