The stock image captioning application in Joomla! 1.5, caption.js, modifies the DOM of the page in ways that are not standards-compliant. The application inserts a <div> inside of a <p>, which obviously isn't permitted. This leads to unpredictable page rendering, and outright chokes Konqueror 3.5.10. Also, the existing application doesn't work for images inside of a hyperlink, which is downright silly, IMO.
I have rewritten caption.js to produce a valid structure using nested span tags. This nesting also permits nice styling of captioned image borders.
I have tested this in the current versions of IE, FF, Safari, Konqueror, Opera, and Chrome. I do not have access to any boxes still running IE 6, so I have not tested it on IE 6. I'm not too concerned about supporting IE 6, especially as Microsoft is putting the finishing touches on IE 8. That said, if you have access to IE 6, please let me know if the captioned images look acceptably close to how IE 7 or FF 3 renders them, so I can update this page.