gecko-dev/layout/style/TopLevelImageDocument.css

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/*
  This CSS stylesheet defines the rules to be applied to ImageDocuments that
  are top level (e.g. not iframes).
*/

@media not print {
  body {
    margin: 0;
  }

  img {
    text-align: center;
    position: absolute;
    margin: auto;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
  }

  img.overflowingVertical {
    /* If we're overflowing vertically, we need to set margin-top to
       0.  Otherwise we'll end up trying to vertically center, and end
       up cutting off the top part of the image. */
    margin-top: 0;
  }

  .completeRotation {
    transition: transform 0.3s ease 0s;
  }
}

img {
  image-orientation: from-image;
}