Since you want an external program (the device's browser) to be able to access a file that your app is providing, you need to:
1) Copy it from the .DirAssets to a Shared directory.
like this?
File.Copy(File.DirAssets, "page.html", File.Dirinternal, "page.html")
is it mandatory to copy it? can't I ask the default browser to read the file directly from dirassets?