nested_conditions.mss

@sans_bold: 'Helvetica Neue', 'Foo Bar';
#world[zoom>=11][zoom<=14][reflen<=8][type='bicycle'] {
    [network='ncn'],[network='rcn'] {
        shield-name: "[ref]";
        shield-face-name: @sans_bold;
        shield-file: url(img/shield-motorway-1.png);
        [network='ncn'] {
            [reflen=8] { shield-file: url(img/shield-motorway-8.png); }
        }
        [network='rcn'] {
            [reflen=8] { shield-file: url(img/shield-trunk-8.png); }
        }
    }
}

result.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE Map[]>
<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over" maximum-extent="-20037508.34,-20037508.34,20037508.34,20037508.34">

<FontSet name="fontset-0">
  <Font face-name="Helvetica Neue"/>
  <Font face-name="Foo Bar"/>
</FontSet>
<Style name="world" filter-mode="first" >
  <Rule>
    <MaxScaleDenominator>400000</MaxScaleDenominator>
    <MinScaleDenominator>25000</MinScaleDenominator>
    <Filter>([type] = 'bicycle') and ([network] = 'rcn') and ([reflen] = 8)</Filter>
    <ShieldSymbolizer file="img/shield-trunk-8.png" fontset-name="fontset-0" ><![CDATA[[ref]]]></ShieldSymbolizer>
  </Rule>
  <Rule>
    <MaxScaleDenominator>400000</MaxScaleDenominator>
    <MinScaleDenominator>25000</MinScaleDenominator>
    <Filter>([type] = 'bicycle') and ([network] = 'ncn') and ([reflen] = 8)</Filter>
    <ShieldSymbolizer file="img/shield-motorway-8.png" fontset-name="fontset-0" ><![CDATA[[ref]]]></ShieldSymbolizer>
  </Rule>
  <Rule>
    <MaxScaleDenominator>400000</MaxScaleDenominator>
    <MinScaleDenominator>25000</MinScaleDenominator>
    <Filter>([reflen] &lt;= 8) and ([type] = 'bicycle') and ([network] = 'rcn')</Filter>
    <ShieldSymbolizer fontset-name="fontset-0" file="img/shield-motorway-1.png" ><![CDATA[[ref]]]></ShieldSymbolizer>
  </Rule>
  <Rule>
    <MaxScaleDenominator>400000</MaxScaleDenominator>
    <MinScaleDenominator>25000</MinScaleDenominator>
    <Filter>([reflen] &lt;= 8) and ([type] = 'bicycle') and ([network] = 'ncn')</Filter>
    <ShieldSymbolizer fontset-name="fontset-0" file="img/shield-motorway-1.png" ><![CDATA[[ref]]]></ShieldSymbolizer>
  </Rule>
</Style>
<Layer name="world"
  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
    <StyleName>world</StyleName>
    <Datasource>
       <Parameter name="file"><![CDATA[/Users/tmcw/src/carto/test/rendering/layers/world/931c441e-shape_demo.shp]]></Parameter>
       <Parameter name="type"><![CDATA[shape]]></Parameter>
    </Datasource>
  </Layer>

</Map>