When trying to do background offlining of some pages, they hang during the MHTML serializing step. 

Best current guess is that these websites change very frequently (on the order of every 3-4 seconds), and are rebuilding the DOM tree while we are trying to serialize it, and we make a request to serialize a frame, the frame disappears, and we never get the response back that serialization is done.

One potential fix is to have a timer, and stop the serialization if the time limit is exceeded.

A second potential fix is to stop the running JavaScript before we archive the page, so it cannot change the DOM while we are trying to offline it.

Web pages we have seen this behavior on:
ig.com.br
makemytrip.com


Reportedly the issue is not happening any more on ig.com.br, so perhaps they have fixed the fact that they updated the web page every few seconds, but we should still consider fixing this potential problem area.