JSFiddle - React, Tailwind, and code Playground

HTML

<input class="postalcode" placeholder="Enter your postal code">

CSS

.postalcode::-webkit-input-placeholder {
   text-transform: none;
}

.postalcode:-moz-placeholder { /* Firefox 18- */
   text-transform: none; 
}

.postalcode::-moz-placeholder {  /* Firefox 19+ */
   text-transform: none;
}

.postalcode:-ms-input-placeholder {  
   text-transform: none;  
}

.postalcode {
    text-transform: uppercase;  
}