One more thing : I displayed, then copy-pasted the source code of the page into a blank fresh .html file (i.e. pure html, no php / smarty templates interpretation), adapted the existing “application/javascript” mime types of the page (which are not related to the tweet button at all) to “text/javascript” and displayed the result in my favorite browser… guess what ? the button is displayed correctly…
I then (for test purpose) server-side forced my site to declare content-type as “text/html” instead of “application/xhtml+xml” and again, with no other modification, the button is displayed correctly.
Therefore it must be somehow related to XHTML headers / how DOM is handled when content-type is XHMTL, but I have no clue how to fix it and why I have this bug.