As the title asks,
What does the + sign mean in <script type="text/javascript+protovis">?
Are there any good materials?
Shamelessly stolen from a different answer here.
Pattern for naming mimetypes is as follows:
A dot hierarchically separates multiple "elements" (for instance, config is child of iptv, that is child of nokia, that is child of vnd).
A hyphen separates composite words (as in google-earth and openxmlformats-officedocument).
A plus sign serves to further specify the serializing format (+json and +xml in these examples).
The x- prefix should be used for MIME types not registered with IANA (and, thus, not shown on that list).
Also, text/javascript+protovis is not equivalent to text/javascript. The two could be parsed differently.