QCH fonts are too large
When compared to the Qt documentation, cppreference appears to have really large fonts. And the problem is exacerbated with current Qt5 versions which lack QtWebKit, so one can't even scale down the main text: only the caption scales.
The fonts appear correct in the official QCH book release 20170409, and are already too large in the 20181028 release. And this problem remains with the releases made from this repo.
See the following screenshots for comparison between Qt docs and C++ docs.
QString docs @ Qt5 Assistant:

std::string docs @ Qt5 Assistant:

Even if I open in Qt4 Assistant built with QtWebKit support, the fonts are too large, although the content is easier to scale (and formatting is less broken, but this is another issue, #47).
std::string docs @ Qt4 Assistant:

The problem is that Qt5 Assistant does not support full HTML, only part of it. We currently convert HTML and CSS features that are not supported by Qt to some equivalents that are supported, but this is error prone and could be improved.
Just released QtCreator 4.11 introduced optional litehtml-based help rendering backend that works somewhat better:

It's still not ideal, but still better than before.
You can change the backend in Tools -> Options -> Help -> General -> Behavoiur
Edit I was to quick to judge, the new backend has problems too:
