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:
]]>
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. But the stylesheet has version 1.0 therefore forward compatible processing MUSTN'T be enabled, so instead of reading this you should see a XSLT processing failed message or error.