As defined in the XSLT specification, in section 2.5 :
An element enables forwards-compatible mode for itself, its attributes, its descendants and their attributes if either it is an xsl:stylesheet element whose version attribute is not equal to 1.0. (...)
(...)
If an element is processed in forwards-compatible mode, then:
- if the element has an attribute that XSLT 1.0 does not allow the element to have or if the element has an optional attribute with a value that the XSLT 1.0 does not allow the attribute to have, then the attribute must be ignored.
This document contains the following XSLT stylesheet appended:
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If your browser supports XSLT, and you are reading this document, then your browser ignored a custom attribute from the xsl stylesheet therefore conforming to rule three of forward compatible processing. Otherwise you would have seen a XSLT processing error.