How to use fancybox
fancybox guide

If you want your WordPress site’s gallery using fancybox, but don’t want to use plugins, you can integrate FancyBox into your gallery like this:

WordPress already uses jQuery we’ve got a neat little trick to add FancyBox. Actually it’s just one line of code that we need to use inside our jQuery script calls in the header.
jQuery('.gallery-icon a').addClass('fancybox').attr('rel','gallery');

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