superset2/panoramix/static/widgets/viz_heatmap.css

.heatmap .axis text {
  font: 10px sans-serif;
}

.heatmap .axis path,
.heatmap .axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.heatmap svg {
}

.heatmap canvas, .heatmap img {
  image-rendering: optimizeSpeed;             /* Older versions of FF          */
  image-rendering: -moz-crisp-edges;          /* FF 6.0+                       */
  image-rendering: -webkit-optimize-contrast; /* Safari                        */
  image-rendering: -o-crisp-edges;            /* OS X & Windows Opera (12.02+) */
  image-rendering: pixelated;                 /* Awesome future-browsers       */
  -ms-interpolation-mode: nearest-neighbor;   /* IE                            */
}