Why isn't it supported by default?
To enable the VLC mozilla plugin's support of the mime type, video/divx, all you need to do is modify the file, mozilla/vlcplugin.h and add the lines,
/* DivX */ \
"video/divx:div,divx:DivX video;" \
to the definition, PLUGIN_MIMETYPES, then run make and make install. Then this allows the VLC plugin to handle the HTML embed type="video/divx".
Since DivX have not released a Web Player plugin for *nix users (only Windows and Mac), I think this would be nice to include in a future release of VLC.