non-standard attributes html tags. good thing? bad thing? your thoughts?

html (or maybe only xhtml?) comparatively despotic comes non-standard attributes tags. aren't biased spec, following your formula deliberate non-compliant.

non-standard attributes sincerely useful flitting along meta-data javascript however. instance, couple suspect arrangement popup, set name popup an attribute:

<a href="#null" class="popup" title="see popup!" 
   popup_title="title popup">click me</a>

alternatively, store due popup dark element, span:

<style>
    .popup .title { display: none; }
</style>
<a href="#null" title="see popup!" class="popup">
    click me
    <span class="title">title popup</span>
</a>

i am ripped however should comparison method. initial slight some-more obvious and, i'm guessing, doesn't screw hunt engines shade readers much. conversely, second choice creates storing infinite amounts information easier thus, some-more versatile. also standards compliant.

i am extraordinary communities thoughts are. hoop conditions this? does probity initial slight comparison appetite downsides (if there any)?