Function (+ accompanying css) for creating "cover"-like background images with "responsive" srcset via wp_get_attachment_image

bg-img-cover.php

<?php
  function bg_img_cover($id, $size, $focus = null, $classes = null, $atts = []) {
    if ( !empty($focus) ) {
      $atts['style'] = "top: {$focus}; -webkit-transform: translate( -50%, -{$focus}); transform: translate( -50%, -{$focus});";
    }
    $img_src = wp_get_attachment_image($id, $size, false, $atts);
    if( !empty($classes) ) {
      $img_src = str_replace( 'class="', "class=\"{$classes} ", $img_src );
    }
   return $img_src;
  }
?>
<style>
  .bg-img-cover {
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    overflow: hidden;
  }
  .bg-img-cover img {
    position: absolute;
    top: 50%;
    left: 50%;
    width: auto;
    max-height: none;
    max-width: none;
    min-height: 100%;
    min-width: 100%;
    transform: translate(-50%,-50%);
    height: auto;
  }
</style>
<div class="bg-img-cover">
<?php
  $img_id     = (int)get_field( 'img_id' );
  $focal_pt   = (int)get_field( 'img_focal_pt' ) . '%';
  $classes    = 'classname';
  $other_atts = ['alt'=>'Some alt text perhaps?'];

  echo bg_img_cover($img_id, 'size', $focal_pt, $classes, $other_atts);
?>
</div>