[statictype]

Why not just add arbitrary attributes to HTML nodes (before the data- attributes were standardized)?

Is there some reason not to do that besides 'it would fail a validator test'?

[kaneo]

The attributes you decide to use now may wind up in the HTML spec some day. If that were to happen, your site may break.

...I hear that a bit when this question comes up. Personally, I think the site would be due for an update by then anyway.