The specification appears to contain no clear statement that using an undefined element in the XSLT namespace is a static error.
It's clear that it's NOT a static error in forwards compatibility mode, but otherwise, there's no explicit statement.
The definition of error XTSE0010 almost says it, but not quite.
Note that the term "XSLT element" is defined as "an element in the XSLT namespace defined in this specification"; an element in the XSLT namespace that's not defined in the specification is therefore technically not an XSLT element.
The specification appears to contain no clear statement that using an undefined element in the XSLT namespace is a static error.
It's clear that it's NOT a static error in forwards compatibility mode, but otherwise, there's no explicit statement.
The definition of error XTSE0010 almost says it, but not quite.
Note that the term "XSLT element" is defined as "an element in the XSLT namespace defined in this specification"; an element in the XSLT namespace that's not defined in the specification is therefore technically not an XSLT element.