Certain TinyMCE plugins scan for all spans in the document and remove any that do not have classes (or a handful of other attributes). This is quite aggressive, and breaks the regex used to scan for MCE views when transforming them back to plain text. Adding a class ensures that any plugins that do so will not break MCE views.
see #21812, #21390.
git-svn-id: http://core.svn.wordpress.org/trunk@22162 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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