Oneboxing is a handy feature in chat for quickly previewing links, but the list of supported sites is very small, and occasionally boxes for sites such as Wikipedia can break and fetch any image/text.
Instead of having to request additional sites to be added to the onebox list (1, 2, 3, 4, 5, and of course this huge list), we could let the sites themselves handle the oneboxing with some
<meta> tags, such as:
<meta name="onebox-title" content="The Onebox Title"/> <meta name="onebox-image" content="/image.png"/> <meta name="onebox-summary" content="This is the summary for a site's onebox"/>
Then when a link is placed on its own in chat, the oneboxing spider would look for these tags and, if they are found, show the appropriate data.
Support for these tags would certainly be bad (for lack of any real better term) at first, though most users of SO chat could easily add it to their personal / company's sites (should they be in such a position) and writing friendly suggestions to other sites to implement them isn't exactly a gargantuan effort.
This would also present an opportunity for currently oneboxed sites to set in stone what should be shown, such as each Wikipedia article setting the main image to be oneboxed instead of SO's spider picking one willy-nilly.
Finally, sites implementing these tags would lay the foundation for other sites and services to develop similar features to the onebox, without having to write per-site scrapers.