Obsidian 0.10.2 on Windows 10:
When I copy a PDF file to my vault, I can create a link to that file and display it inside Obsidian, using the [[filename.pdf]] or [pdf](filename.pdf) syntax. However, when I try to do this with an HTML file, i.e. [[filename.html]] or [html](filename.html), then this does not work. Instead of showing the HTML file, as expected, Obsidian creates an empty filename.html.md file and opens that.
we never supported this. You can try the notation [html](file:///path/to/file.html).
You can try the notation [html](file:///path/to/file.html).
This is not a replacement for internal links, since this notation does not allow relative paths. So as soon as you move your vault elsewhere, these links will all break.
We never supported this.
I considered it a bug, since the behavior is not consistent, even if it never worked. But you’re right, strictly speaking it’s not a bug, so raised this as a feature request now.
I have alredy pointed you to the relevant feature request. I will merge them.
Sorry - this multitasking between Christmas preparations and writing forum posts is not good for the quality of my posts. Should have read and researched the forum better before writing. Anyway, have a great Christmas and thanks for all your work for Obsidian and its community. 
It’s ok! merry christmas!!