JSFiddle - React, Tailwind, and code Playground

HTML

<input width="100%" placeholder="The color should change on focus, but it should be a 300ms transition, not a jarring change." >

CSS

::-webkit-input-placeholder { color: inherit; opacity: 1; font-weight: normal !important; -webkit-transition: color 0.3s; transition: color 0.3s; /* WebKit */ }
:-moz-placeholder { color: inherit; opacity: 1; font-weight: normal !important; -moz-transition: color 0.3s; transition: color 0.3s; /* Firefox 4 to 18 */ }
::-moz-placeholder { color: inherit; opacity: 1; font-weight: normal !important; -moz-transition: color 0.3s; transition: color 0.3s; /* Firefox 19+ */ }
:-ms-input-placeholder { color: inherit; opacity: 1; font-weight: normal !important; -ms-transition: color 0.3s; transition: color 0.3s; /* Internet Explorer 10+ */ }

:focus::-webkit-input-placeholder { color: rgb(80%, 80%, 80%); /* WebKit */ }
:focus:-moz-placeholder { color: rgb(80%, 80%, 80%); /* Firefox 4 to 18 */ }
:focus::-moz-placeholder { color: rgb(80%, 80%, 80%); /* Firefox 19+ */ }
:focus:-ms-input-placeholder { color: rgb(80%, 80%, 80%); /* Internet Explorer 10+ */ }

input { width: 100%; display: bock; font-size: 18pt; }