Just implemented basic HTML and accessibility validation in Kitten¹ (using html-validate²).
For now, validation results are written out to the Node.js console. I do have a few ideas about how to improve that in the future, though :)
PS. The image validation errors you see are from posts coming in from my Mastodon federated timeline with images that don’t have alt text. Please folks, write text descriptions for your images.
¹ https://codeberg.org/kitten/app
² https://html-validate.org/