在react中使用pseudo-class样式时使用类名而不是样式属性
https://www.w3schools.com/howto/howto_css_placeholder.asp
因此：
App.jsx

<input className="my-input" placeholder="Search here..." />

styles.css

.my-input::placeholder { /* Chrome, Firefox, Opera, Safari 10.1+ */
  color: red;
  opacity: 1; /* Firefox */
}

.my-input:-ms-input-placeholder { /* Internet Explorer 10-11 */
  color: red;
}

.my-input::-ms-input-placeholder { /* Microsoft Edge */
  color: red;
}