RadEditor has a set of content filters that modify the content in various ways. You can find more detailed information on the subject in the following help article and live demo:
Regarding the <span> to <font> conversion, this modification is applied to the content when editor is DesignMode because the browser's commands used by RadEditor utilize <font> tags and does not recognize <span> tags correctly.
As to the client-side API, the get_html() client-side method of the editor can provide the original content (thats not being processed by the content filters) when the provided parameter is False and converted content when the parameter is True (see this article
In addition, since RadEditor is built on top of the browser's RichTextEditing engine each browser may apply various changes to the content when added to the control, i.e. earlier versions of InternetExplorer convert all HTML tags in upper case.
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