It is looks like W3C\'s validator return a validation error on prettyPhoto\'s rel attribute for HTML5 pages. How do I solve this error?
Bad value prettyP
Using rel attribute with non-proposed (thus not allowed) values not valid for HTML5 markup. Value prettyPhoto is not in the list of proposed values. So you may face the difficulties with getting your web-page with image gallery passing validation.
A Possible Solution:
Open jquery.prettyPhoto.js (presumably non-minified one) and perform find & replace function of your text-editor:
replace all occurrences of attr('rel') with attr('data-gal').
In your gallery code use:data-gal="prettyPhoto[galname]"
instead of:
rel="prettyPhoto[galname]"
Initialize your prettyPhoto with:
jQuery("a[data-gal^='prettyPhoto']").prettyPhoto();
And you are on the right way for getting your code valid!
You can also read this article with this possible solution.