Question

As the title asks,

What does the + sign mean in <script type="text/javascript+protovis">?

Are there any good materials?

Answer 1

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.