An XSLT 1.0 processor handles 2.0 stylesheets in forwards compatibility mode
An XSLT 2.0 processor handles 1.0 stylesheets:
As a 1.0 processor, or
in backwards compatibility mode
A mixed-mode stylesheet uses the appropriate mode.
Although the goal is that a 2.0 processor running a 1.0 stylesheet in backwards compatibility mode should produce the same results as a 1.0 processor, this is not guaranteed to be the case for all stylesheets.