with Ashkan solution anywhere in the page

<style>
 .ui.table thead tr:first-child>th {
 position: sticky !important;
 top: 0;
 z-index: 2;
 }
</style>

<div style="overflow-y: auto; overflow-x: hidden; resize: vertical; height: 400px;">
<table class="ui celled striped selectable table">
...