The use of this word is controversial, as it suggests that XML has optional features, which is not true. There are, however, some actions which an XML processor is allowed but not required to take, particularly in the case when it is a non-validating processor. This will absolutely not weaken XML's claim that any processor can read any document (subject to character encoding issues), and is not expected to cause operational problems; furthermore the spec describes how an application may avoid any unpredictability in results.
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