I opened the Qt 5 porting guide (http://doc-snapshot.qt-project.org/qt5-stable/qtdoc/portingguide.html) in Qt Assistant. The link "Qt Declarative" points to https://qt-project.org/doc/qt-4.8/qtdeclarative.html

When I click on it, I get:

The page could not be found!

Error loading: https://qt-project.org/doc/qt-4.8/qtdeclarative.html

Please make sure that you have all documentation sets installed.

In contrast, clicking on HTTP links launches my browser as expected.