Edit
Posts composed of a pattern that, if cropped correctly and used as a "tiled" background, will repeat seamlessly with no visible edges.
The following JavaScript snippet can be useful to check whether an image is tileable without further cropping. Running it on the post page will replace the page with only the image repeated in a 3×3 grid:
let {fileUrl, width, height} = document.querySelector('.image-container').dataset;
document.body.outerHTML = '<body style="background:url(\''+fileUrl+'\') repeat; min-width:'+(width*3)+'px; min-height:'+(height*3)+'px;"></body>';