Some time ago, I stuck on reading manuals trying to find correct and clear way to enlarge DPI of images rendered by WebKit (in Qt).
If you need to render HTML page in your Qt app, you can use widgets that use
QTextDocument, such as
QTextEdit. But all of them support limited subset of HTML 4 and CSS 2.1 (Qt Project — Supported HTML Subset). But if you need to use some more complicated things (or even some typical CSS styles) excluded from that list, you should use WebKit (WebKit in Qt).