HTML5 has a “custom data attribute”. It lets you to have any attribute/value pairs in a tag. Such attribute has names starting with
data-. Here's a example used by Twitter:
<a href="https://twitter.com/share" class="twitter-share-button" data-via="xah_lee">Tweet</a>
A custom data attribute is an attribute in no namespace whose name starts with the string "data-", has at least one character after the hyphen, is XML-compatible, and contains no uppercase ASCII letters.
Here's one example of using it as accessed date of a site.
<a href="…" data-accessed="2012-09-22">7 Tips You Must Know</a>
When the site went dead, you can add a defunct date, like this:
<a href="…" data-accessed="2012-09-22" data-defunct="2012-07-06">7 Tips You Must Know</a>
You can use it for any purpose you want for your website or online app.