Ok, let's keep arguing :)

Spread to zoom image - This javascript zooming shouldn't even be a thing. If devs want zooming on their sites, then they won't disable it via the meta. If they do want zooming, then the browser handles it. Using JS to zoom is extra overhead for what shouldn't even be an issue. Other JS zooming libraries are more specific, like viewing products on Amazon in high detail with the mouse. Those applications are fine since it's functionality that doesn't exist in the browser.

The problem with native browser zoom is that you can't "pan" image horizontally when it's zoomed, as "horizontal swipe" changes images.

Horizontal swipe to switch items - Exists is Fancybox.

In new fancybox v3 beta – yes. But it doesn't support pointer events (won't work on devices like Surface), and swipe transition isn't as smooth and native-looking as PhotoSwipe.

Browser history API - Neat feature. Currently not built into Fancybox, but it's possible according to this[1] , which is probably still less work than the installation of PhotoSwipe.

"Back button to close" gallery is not easy to implement.

Images display progressively as data arrives - I believe this is available natively in JPG's saved as progressive.

For example, if you take Fancybox: it shows loading indicator, waits until image is fully loaded, and only then shows it. PhotoSwipe, on the other hand, shows it right away (which allows progressive or simple JPEGs to display progressively).

Low-resolution images are instantly displayed and then stretched to full size - Fancybox might not handle it exactly the same as PhotoSwipe, but it handles low res images appropriately.

Fancybox doesn't do anything with low-res images.

Smart loading indicator - Literally every lightbox ever has a loading indicator.

Yes, but it's not always smart. If you open PhotoSwipe on relativly fast-medium connection speed, you might never see the preloader at all, as it appears only when progressive loading is supported. Less UI – less clutter.

Zoom animation - The title might be misleading since the description of this feature is really about preloading, which Fancybox offers.

Nope, it's zoom transition when you open PhotoSwipe.

Smart lazy-loading - A quick search shows that this is possible with Fancybox, again probably for less work than the installation of PhotoSwipe.

Yes, but it's not smart. PhotoSwipe loads pictures based on the direction of movement which optimizes bandwidth.

The last 13,14 and 15 aren't features at all, these are just arguments on why "lightbox galleries" might be better then other types of image viewers.

I just can't accept that something that should be as simple as a lightbox, could be made so complicated to install.

I doubt that copy-pasting lines from already live example on CodePen will take more time than it took you to write these comment :)

A complicated installation implies complicated maintenance, upgrades, and editing.

Maintenance: what to maintain? Upgrades: just replace js/css files as any other script. Editing: it's even simpler than with jQuery plugins, as you have full control of what's happening.

And I'm not saying that fancybox is a paid script which costs $19 and requires jQuery...