These packages are experimental (especially these with infixes like -qtwebengine or -qtwebkitng) and may work incorrectly or may not work at all.
If you are Windows user then you should prefer installers, especially in case of 64 bit Windows 8 or newer.
In case of errors (crashes, incorrect rendering) please refer to issue trackers:
https://bugreports.qt-project.org/issues/?jql=project%20%3D%20QTWEBKIT%20ORDER%20BY%20key%20DESC
https://github.com/Emdek/otter/issues?labels=upstream+issue&page=1&state=closed
Preferred way to obtain binaries is to build own using Qt SDK 5.3.0 or later (5.5 is recommended), it can be obtained here:
http://www.qt.io/download-open-source/
More details can be found in INSTALL file in source package.
See wiki page for list of all available packages:
https://github.com/Emdek/otter/wiki/Packages