register(rule) {
    this.map[rule.key] = rule
    if (rule instanceof StyleRule) {
      this.map[rule.selector] = rule
      if (rule.id) this.classes[rule.key] = rule.id
    } else if (rule instanceof KeyframesRule && this.keyframes) {
      this.keyframes[rule.name] = rule.id
    }
  }
Register a rule in `.map`, `.classes` and `.keyframes` maps.