example.css

/** bubble visualization */

#earthquakes{
  marker-opacity: 0.9;
  marker-line-color: #FFF;
  marker-line-width: 0.5;
  marker-line-opacity: 1;
  marker-placement: point;
  marker-multi-policy: largest;
  marker-type: ellipse;
  marker-fill: #FFCC00;
  marker-allow-overlap: true;
  marker-clip: false;
  marker-comp-op: multiply;
}
#earthquakes [ mmio <= 12] {
   marker-width: 24.0;
  [zoom>5]{marker-width: 48;}
}
#earthquakes [ mmio <= 11] {
   marker-width: 16.3;
  [zoom>5]{marker-width: 32;}
}
#earthquakes [ mmio <= 9.5] {
   marker-width: 8.7;
  [zoom>5]{marker-width: 18;}
}
#earthquakes [ mmio <= 6.5] {
   marker-width: 1.0;
  [zoom>5]{marker-width: 2;}
  [zoom>6]{marker-width: 4;}
  [zoom>7]{marker-width: 8;}
  marker-line-width: 0;
}
#earthquakes [ mmio <= 3] {
   marker-width: 0.5;
  [zoom>5]{marker-width: 1;}
  [zoom>6]{marker-width: 2;}
  [zoom>7]{marker-width: 4;}
}

#earthquakes::labels 
[mmio>=10][zoom>6],
[mmio>=8][zoom>7],
[mmio>=6][zoom>8],
[mmio>=4][zoom>9]
{
  text-name: [mmio];
  text-face-name: 'DejaVu Sans Book';
  text-size: 21;
  text-label-position-tolerance: 10;
  text-fill: #000;
  text-halo-fill: rgba(255,255,255,0.8);
  text-halo-radius: 2	;
  text-dy: 0;
  text-allow-overlap: true;
  text-placement: point;
  text-placement-type: simple;
}