gradient img border

<style>
#gradient {
border: 3px solid;
border-image-slice: 1;
border-image-source: linear-gradient(#000, #ccc);
}
</style>

<div id="gradient"><img src=https://www.rover.com/blog/wp-content/uploads/2019/06/cat-4223305_1920-300x200.jpg></div>