There is a bug with Markdown and umlaut characters (and probably all utf-8 chars). If you use an umlaut inside of a Header, the markdown library will add an ID with that umlaut to the header tag. This causes invalid HTML.
Project File: Markdown Umlauts.rw8.zip - Droplr
This is valid HTML5 (I ran your example project through an HTML5 validator to be sure).
My guess is you’re validating against HTML 4, but I could be wrong - where are you seeing validation errors?
Well I feel like an idiot. Sometimes when I am in support mode, I forget to stop and think a bit more.
The customer stated that it was failing some SEO Check but I do not know which one. I asked today.