Best practice: class or data attribute as identifier

If you merely want to associate "values" to DOM elements for computational purposes then data-attribute is the way to go since,

anything that is "data-" will be treated as a storage area for private data (private in the sense that the end user can’t see it – it doesn't affect layout or presentation)

Also, jQuery provides the .data(), which makes life easier so you don't have the trouble of using the [data-*] selector.

If you are providing a class name then, considering semantics, it should have some style associated with it.

John Resig has written about data-attributes

Best way to test these things is to search for a test suite on jsperf.com

Here is the one you are interested in: http://jsperf.com/id-vs-class-vs-data-attribute

The most efficient selector is to use classes.

This is probably a consequence of classes being used more for selection, so browsers will optimize that more?

According to W3C

data-*

Custom data attributes are intended to store custom data private to the page or application, for which there are no more appropriate attributes or elements. These attributes are not intended for use by software that is independent of the site that uses the attributes.

class

The class attribute has several roles in HTML: As a style sheet selector (when an author wishes to assign style information to a set of elements). For general purpose processing by user agents.

The bold text above is the autoritative assurance that it is ok to use class attribute without its definition in CSS. The warnings from VS 2012 about that are overzealous.

If you use class attribute, you can benefit from native getElementsByClassName searching (with O(1) time complexity) and classList object for toggling, adding and removing class. There's nothing like getElementsByAttributeValue. There is relatively slower Element.querySelectorAll('[data-attr="value"]') ref _{See Oliver Moran's comment}. It has O(n) time complexity.

On the other hand, if you need to store multiple data, you can use dataset attribute. So if you want searching or if the data affect the look of the element, I would use class. If you need to store multiple data, the data would be more appropriate.

In your particular case I would consider required or pattern input attribute (since HTML5 most of input validation moved from JS to HTML). To style such elements, CSS selectors use the same syntax as querySelectorAll.