A semantic-annotations element can be added to rich texts in plugin.xml like this:
<cms:metadata name="content" type="rich-text">
<label i18n="true">CONTENT_TYPE_SIMPLE_CONTENT</label>
<description i18n="true">CONTENT_TYPE_SIMPLE_CONTENT_DESC</description>
<semAnnotations>
<semAnnotation name="penibilite">
<label i18n="true">CONTENT_TYPE_PENIBILITE</label>
<description i18n="true">CONTENT_TYPE_PENIBILITE_DESC</description>
</semAnnotation>
<semAnnotation name="penibilite2">
<label i18n="true">CONTENT_TYPE_PENIBILITE2</label>
<description i18n="true">CONTENT_TYPE_PENIBILITE2_DESC</description>
</semAnnotation>
</semAnnotations>
<validation>
<mandatory/>
</validation>
</cms:metadata>

This element can be saxed on the client.