I came across this [ejs-lint]
Thanks for the pointer, but that wasn’t my suggestion. We do already lint KS macros, and use ejs-lint to do so. But KS macros generally work by building some HTML, which then gets inserted into the document. If we were able to test that the HTML is well-formed, we might be able to find bugs like All pages using `AddonSidebar` display content within `<strong>` · Issue #769 · mdn/kumascript · GitHub.
The nearest I found was GitHub - validator/validator: Nu Html Checker – Helps you catch problems in your HTML/CSS/SVG, but it seems like it would be tricky to get that working in our environment.