Huh, that's a bummer! I only used it once myself to send colleagues draft versions of some markdown file that would later go on our blog, maybe it somehow helped that the source was markdown instead of html? Not sure, I'm sorry to hear of this disappointing experience :/
> maybe it somehow helped that the source was markdown instead of html?
That's probably it. Most of my issues were related to CSS, which markdown does not have[0]. The duplicated headline wasn't CSS though, that is by far the oddest issue. I'll probably file a bug report to pandoc after I replace all the text in this document with lorem ipsum (it is a document for work, so I can't share it publicly in its current form).
[0] unless you embedded your own CSS since markdown technically allows arbitrary HTML
For context: the document I am attempting to process has no javascript. It is a simple Emacs Org document (similar to markdown) rendered to HTML and then processed with pandoc. The only external content was a couple of images.