XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (400 page)

(
before deciding whether a keyword such as
text
is to be interpreted as a
NameTest
or as a
KindTest
.

There are three ways you can select processing instructions. The simple test is
processing-instruction()
, which selects any processing instruction node regardless of its name. If you want to select processing instructions named
xml-stylesheet
, say, then you can write either
processing-instruction(“xml-stylesheet”)
or
processing-instruction(xml-stylesheet)
. The two are equivalent: the syntax with quotes is retained for compatibility with XPath 1.0, while the syntax without quotes is introduced for symmetry with the
element(…)
and
attribute(…)
tests described in Chapter 11.