For those of you who may not know, I have an open source project for SharePoint called SPEasyForms. But this post is not about that, it’s about a general problem you might encounter in SharePoint 2019, which is that none of your CSS files in document libraries work. It just so happens that I first heard about this issue this week, when somebody reported SPEasyForms doesn’t work on SharePoint 2019. I had tested it in 2019 Preview, and it had worked just fine, but the preview license had expired, so I had to spend a few days standing up a 2019 RTM farm. As soon as I did that, I saw the same results as had been reported (i.e. SPEasyForms looked like crap). So I’m just using SPEasyForms to demonstrate the problem, and I’ll go on to talk about how to fix it.
Understand that this problem will affect all CSS files that are loaded out of document libraries. That includes OOB style sheets loaded out of the master page gallery. It does not affect all SharePoint 2019 installations. Like I said, my preview didn’t display this behavior, and that’s not because this only affects the RTM release. Others had already reported this issue with the Preview release. And Microsoft is aware of the problem, but nobody has explained what alignment of the stars will cause this issue, just some people have it and others don’t.