Flash doesn’t support most of the html elements of a standard web page. TinyMCE’s creators Moxiecode have thankfully given us the TinyMCE rules for flash here:

http://wiki.moxiecode.com/index.php/TinyMCE:Configuration/valid_elements#Full_FlashMX_2004_rule_set:

So to get Umbraco & TinyMCE working to give your flash application the markup that it needs you need to modify your \config\tinyMceConfig.config file:

<validElements>
    <![CDATA[+a[href|target],
+b,
+br,
+font[color|face|size],
+img[src|id|width|height|align|hspace|vspace],
+i,
+li,
+p[align|class],
+h1,
+h2,
+h3,
+h4,
+h5,
+h6,
+span[class],
+textformat[blockindent|indent|leading|leftmargin|rightmargin|tabstops],
+u
]]>
</validElements>

Then there’s the issue that flash treats real line breaks and <br/> tags exactly the same! This will probably cause you a few headaches trying to work that out. So to save you that headache, you need to output the rich text markup via XSLT to flash using the XSLT function: normalize-space

Something like:

<xsl:value-of select="normalize-space($myRichTextValue)" />