Html – Lightbox gallery rel with array parameter validation

Ignore it.

If that's the only error you have, then your document is valid HTML5.

Here's what the official (in development) spec states about the <meta> tag: Extensions to the predefined set of metadata names may be registered. I can't find the area in the spec that talks about the "ref" tag values, but the validator treats them similarly (one for links, one for strings), and points us to the extension Wiki. You 'may' register them, but don't have to. In RFC terminology this is a SHOULD not a MUST.

The spec doesn't seem to mandate a fixed list, or use of the Wiki. Doing so would seem odd, as these fields have often evolved with time. It does state that Conformance checkers must use the information given on the WHATWG Wiki MetaExtensions page to establish if a value is allowed or not: values defined in this specification or marked as "proposed" or "ratified" must be accepted. which is an interesting line as it is a specification for the HTML Validators, not HTML5 itself, and doesn't itself make the markup invalid.

In fact, many of these "extensions" are already in the wiki (including your one), they just haven't been accepted. Same with many meta tags, even very common ones. It seems many won't be accepted either.

I think it's very nice of the W3C to create a standardised list of these. It helps developers know what they should be using now and in the future (and can hopefully clean up some things linke reducing the number of ways you can specify a creation date from 5+ to 1).

Unfortunately we are dealing with third parties here (e.g. Apple) – and unless you want to contact every third party who has created one of these informal specification, and tell them to formalize a spec, and submit it to the W3C's list (which may or may not get accepted) what are you to do? At the end of the day you still need to support it.

Anyway, isn't the very point of having these HTML elements to support extensions so vendors don't break the spec by adding new elements to do what the need?

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:

1. 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').

2. In your gallery code use:
   
   data-gal="prettyPhoto[galname]"
   
   instead of:
   
   rel="prettyPhoto[galname]"

3. 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.