"i really don't know if this a topic to deal with, but for my opinion it looks like garbage, if xml-content is enriched with html-markup."
I'll just enjoy that comment for a moment.
Ok, one option is to switch to summary display for syndication: Options > Reading, Syndication Feeds.
Another is to modify your various feed templates to use
the_content_rss() instead of
the_content() for full text feeds. For example, in wp-rss2.php (RSS 2), look for this line:
<content:encoded><![CDATA[<?php the_content('', 0, '') ?>]]></content:encoded>
which I suggest changing to:
<content:encoded><![CDATA[<?php the_content_rss('', false, '', 0, 2) ?>]]></content:encoded>
wp_content_rss() and its parameters: