如何使用 CSS 设置只读属性的样式?

I'm currently using readonly="readonly" to disable fields. I'm now trying to style the attribute using CSS. I've tried using

input[readonly] {
  /* styling info here */
}

但由于某种原因它无法正常工作.我也试过了

input[readonly='readonly'] {
  /* styling info here */
}

input[readonly]
{
    background-color:blue;
}