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<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.

//* Function to display space separated (default) category slugs
function the_category_unlinked( $separator = ' ' ) {
    $categories = get_the_category();
    
    $thelist = '';

    foreach( $categories as $category ) { // concatenate
        $thelist .= $separator . $category->category_nicename;
    }
    
    echo $thelist;
}

<!-- Usage: -->

<div class="post<?php the_category_unlinked(' '); ?>">