Currently, when a special element (like an image) is added to the Swing HTML document, a space character is used as a model placeholder. This choice is not appropriate because space characters have well-known semantic, for example, a space-based BreakIterator could find a non-existent breakpoint in the document.

"The Use of Unicode with Markup Languages" document (UTR#20) recomments that the 'OBJECT REPLACEMENT CHARACTER' (U+FFFC) should be used as a model placeholder for external objects. Swing HTML should follow this recommendation.