Here is a link to the conference paper on MashupOS as discussed in class on Wednesday, which goes into more detail on the <sandbox> and <opensandbox> elements:
Protection and Communication Abstractions for Web Browsers in MashupOS
Helen J. Wang, Xiaofeng Fan, Jon Howell, and Collin Jackson
In Proc. of the 21st ACM Symposium on Operating Systems Principles (SOSP 2007)