interdependant dropdown lists

I want do build a sytem of two dropdown lists in which the result of
the first selection will determine which set of values is proposed in
the second one.

I cannot catch the value of the first selection in a variable, because
it needs to be changed by the user.,
So I chose to use a key and its parameter.

<xsl:key name="GrouperParNbAttr"
match="requetes/requetespredef/requete"
use="count(attributes/attribute)"/>
   <xsl:param name="reqid" select="@id" />

This must give me the dropdown list of all requests which have a
certain number of conditions, and the id of these requests as
parameters

<form name="selectreq">
        <select size="1" name="requete">
      <xsl:for-each select="key('GrouperParNbAttr',1 )">
            <option> <xsl:value-of select="name"/></option>
      </xsl:for-each>
        </select>
        </form>

Once the user has chosen a request by clicking on it,  its id should be
displayed:

<td ><font size="1" >
<b>ID </b>  <xsl:value-of select="$reqid"/>
</font></td>

..and the user must be given the choice between the conditions that
correspond to the request.