<!--rainbow hr-->
<style>
hr.rainbow{
height: 1px; /*for me 1px fits perfect.*/
outline: none;
border: none;
background: linear-gradient(90deg, rgb(255,0,0) 0%, rgb(255,184,0) 12%, rgb(194,255,0) 26%, rgb(0,255,188) 42%, rgb(0,151,255) 57%, rgb(21,0,255) 72%, rgb(201,0,250) 87%, rgb(255,0,138) 100%);
}
</style>
<body>
<hr class="rainbow">
</body>