[Solved]-Correct approach to apply HTML style in x-axis title-Highcharts

Check xAxis.labels.useHTML, It allows us to use html property in the labels

 title: {
  useHTML: true, //added this
  enabled: true,
  text: "<p>Custom with <span style='color:blue'><b>simple</b></span> <i>markup</i></p>",
  style: {
    fontWeight: 'normal'
  }
}

Fiddle demo