Different rendering in latex doc between Windows, macOS and GNU/Linux

[Angelo Graziosi]

Just for completeness..

Using both on Windows, macOS and GNU/Linux builds "emacs -q", I notice that with

  \emph{foo}

'foo' is rendered in 'italic' on Windows and macOS while it is rendered in 
'underlined bold' on GNU/Linux. I tried with different fonts on both systems. 
The color is the same.

Why the behavior is different on GNU/Linux?

Ciao,
  Angelo.