Use attr instead of style to provide defaults and facilitate overrides.

[mbostock]

This will allow us to define a default fill and stroke for ticks and the domain path, rather than requiring users to include these style rules:

.axis line {
  stroke: #000;
}

.axis path {
  fill: none;
  stroke: #000;
}

(I’m not 100% certain crispEdges should be included by default; it’s probably best to use make the defaults as minimal as possible, both for performance and to facilitate overrides.)

Related d3/d3#2102; this would also apply to the text-anchor used to position axis ticks.