You could use the disable-output-escaping attribute on <xsl:value-of>:
<xsl:value-of select="teaser" disable-output-escaping="yes" />
The disable-output-escaping attribute disables the usual escaping of
the XML-significant characters & and <, instead writing them literally
to the result. Note that disable-output-escaping won't be honoured in
all situations, but you might get lucky.
This seems like something of a persistent thorn, what with the
increasing use of RSS which has this particular problem all over it, and
of course bad design decisions elsewhere, that it would might be nice to
have an exsl: extension function for unescaping text, with fallback
possibilities in cases of the unescaped tree being malformed.
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list