In a reply to a post I made the other day Dimitre pointed out, correctly of course, that I had given an XSLT 2 answer to an XSLT 1 question.
However, he also upbraided me for posting an answer that hadn’t been tested. I had in fact tested it and even though the version attribute was set to “1.0” and I used an XSLT 2 replace function every worked with errors or warnings.
So this raised a question – what does the version attribute mean if it doesn’t restrict the language to a particular version?
I did try reading the w3 spec but my eyes started to bleed.
FWIW: I use Oxygen and Saxon 9.3 EE
Firstly, the XSLT specification says how an XSLT processor interprets the version attribute, but it doesn’t constrain what pieces of software other than an XSLT processor do with it. For example, an IDE (such as XML Spy) might look at the version attribute and use it to decide whether to launch an XSLT 1.0 or XSLT 2.0 processor. Once an XSLT 1.0 or 2.0 processor is launched, its behaviour is controlled by the relevant spec.
What an XSLT 1.0 processor does with the version attribute is defined by the XSLT 1.0 specification; what a 2.0 processor does is defined by the XSLT 2.0 spec.
The XSLT 1.0 spec says that if the version is NOT 1.0, the processor runs in forwards compatibility mode. This basically means that it does its best to ignore constructs that aren’t defined in the 1.0 spec. So if your stylesheet says version=”2.0″, and you run it with a 1.0 processor, then an attribute that is new in 2.0 like xsl:sort/@collation will be ignored. An instruction that isn’t recognized causes a failure only if it is actually executed, and if it has no xsl:fallback child instruction to give a fallback behaviour for 1.0 processors. The design principle is that using 2.0 constructs should not cause a 1.0 processor to fail; wherever possible, it should cause it to run with some kind of fallback behaviour.
The XSLT 2.0 specification (which controls the behaviour of a 2.0 processor) distinguishes verion<2.0, version=2.0, and version>2.0. When version<2.0, the processor runs in “backwards compatible mode”. This doesn’t mean that 2.0 constructs are rejected; rather it means that 1.0 constructs are executed with semantics as close as possible to those defined in the 1.0 specification. For example, all arithmetic is carried out as double floating point, even if the operands are decimals. When version>2.0, the processor runs in forwards compatible mode, which is very like forwards compatible mode in the 1.0 specification: it means that if you use XSLT 3.0 constructs, the processor will do its best to ignore them or execute fallback instructions.