The Qt sourcecode contains large numbers of broken PNG images. These cause warnings from libpng to be printed to stderr when running any Qt application. Simply creating a QWebView and settings its URL to about:blank causes such a warning, for example:

python -c "from PyQt5.Qt import *; app = QApplication([]); s = QWebView(); s.setUrl(QUrl('about:blank'))"
libpng warning: iCCP: Not recognizing known sRGB profile that has been edited

A list of such broken PNG images in the source tree can be generated like this:

find . -name '*.png' -exec identify {} \; | grep MagickPNGWarningHandler

Most of these warnings can be fixed by simply removing the broken embedded color profile like this:

convert file.png -strip fixedfile.png