toJSON() {
    const json = {}
    for (const prop in this.style) {
      const value = this.style[prop]
      if (typeof value !== 'object') json[prop] = value
      else if (Array.isArray(value)) json[prop] = toCssValue(value)
    }
    return json
  }
Returns JSON representation of the rule.
Fallbacks are not supported.
Useful for inline styles.