When the user agent claims to support facilities defined within this specification or required by this specification through normative reference, it must do so in ways consistent with the facilities' definition. If a user agent encounters an element it does not recognize, it must continue to process the children of that element. If the content is text, the text must be presented to the user. If a user agent encounters an attribute value it doesn't recognize, it must use the default attribute value. When rendering content, user agents that encounter characters or character entity references that are recognized but not renderable should display the document in such a way that it is obvious to the user that normal rendering has not taken place.
White space is handled according to the following rules.
The XML processor normalizes different systems' line end codes into one single LINE FEED character, that is passed up to the application. Comments are removed entirely and do not affect white space handling. One white space character on either side of a comment is treated as two white space characters. The choice of the resulting character is user agent dependent and is conditioned by the script property of the characters preceding and following the LINE FEED character. A sequence of white space characters with one or more LINE FEED characters must be reduced in the same way as a single LINE FEED character.