> As a (web app) developer I never quite sure what to put in alt.

Are you making these five mistakes when writing alt text? [1] Images tutorial [2] Alternative Text [3]

[1]: https://www.a11yproject.com/posts/are-you-making-these-five-...

[2]: https://www.w3.org/WAI/tutorials/images/

[3]: https://webaim.org/techniques/alttext/