There are two classes in Qt that can display HTML content: QTextBrowser and QWebView. They seem similar, but in fact they are quite different. The most obvious difference is that QTextBrowser is part of QtGui, while QWebView belongs to the QtWebKit module. It's a pretty big library, about 12.5 MB (32-bit Windows DLL), which is more than QtCore and QtGui combined. I'm going to try to explain why there's such difference.
Although QTextBrowser can display a piece of HTML content, technically it's a rich text viewer, not an HTML viewer. It may seems like the same thing, and in fact there are many similarities. A rich text document consists of blocks of text (similar to HTML paragraphs) and frames (similar to HTML div's). It also supports tables, lists and images. However, the layout of the rich text document is much simpler than that of HTML. The text of the document simply flows from top to bottom. There is no concept of absolute positioning, floating frames, etc. You can forget about tableless layouts and most CSS styles. Even the margin and padding settings are not always respected and it takes some experimentation to get the spacing right.
The QTextBrowser can export the rich text in HTML format, and import it back without loosing information. However, this is not fully standard compliant HTML, and when opened in a regular browser, it may not necessarily look the same as in the QTextBrowser. What's worse, although HTML can be imported from an external source, only a limited subset of HTML tags, attributes and CSS properties will be recognized, and the document will almost surely look very different than in a browser.
Side note: The difference between rich text model and HTML model is not just specific to Qt. Also many common word processors, including MS Word, have a similar limitation. You can import HTML to Word, but the layout of a web page will not be strictly preserved. And because MS Outlook uses the same engine as Word to render HTML emails, it also only supports a limited subset of HTML and CSS. This makes it difficult to create HTML emails which look good in all email clients including Outlook.
This obviously doesn't mean that you shouldn't use QTextBrowser at all. First of all, it's got an excellent cursor-based API which lets you create rich documents very efficiently without writing a single piece of HTML markup. This is excellent for creating various reports, etc. Just ensure that when you make a lot of changes in the document, it shouldn't be connected to the browser, otherwise updating it will be slow. Also don't forget that rich text is also supported out-of-the by many other widgets, for example QLabel or QToolTip. Not to mention that you can also edit rich text using the QTextEdit widget (which is, in fact, inherited by QTextBrowser).
QWebView, on the other hand, is a full blown, standards compliant HTML browser. Actually it's based on the same code which powers Chrome and Safari browers. It works natively with HTML and supports all tags, attributes and CSS properties. It also has a built-in JavaScript interpreter. You should definitely use QWebView when you need to display external web content, create complex layouts or use dynamic, scripted content. This comes at the cost of the extra 12.5 MB linked library and slightly higher resource usage. It's hard to measue the difference in performance. Simple documents work very fast in both controls, and complex documents can only be handled well by QWebView.
Despite many advantages, QtWebKit is far from being perfect. In the next part I will write about various bugs and problems I've encountered so far while porting the WebIssues Desktop Client from QTextBrowser to QWebView.