In the attached mathml example, test.html, the integral symbol has small cuts in it and the underbrace does not show up.
I've attached a screenshot comparing WebKit trunk @r133913 and Firefox.
Created attachment 173103 [details]
Created attachment 173104 [details]
It seems that you don't have the STIX fonts installed on your system.
Webkit should probably do as Firefox and use the size variant for the integral and ignore the construction with several parts.
This depends on bug 99614 and bug 72828.
Created attachment 233522 [details]
Here is a screenshot with a trunk build of WebKitGTK+. This does not work well yet on all platforms, this is essentially bug 99614.
*** This bug has been marked as a duplicate of bug 99614 ***