Allow Span to be hidden when visualizing the document. For example, introduce a Span.IsHidden property.

In MS Word, text is hidden using the Home -> Font -> Font -> Effects -> Hidden. In the RTF format, such text is preceded (marked) with a '\v' tag. In OOXML, the tag is <vanish/>

Note: Hidden text is visualized with dotted underline when formatting symbols are shown. Hidden text is not exported to PDF.