I'm finding this problem with QML Text objects, using RichText format, but it may also apply to Qt QTextDocument objects.

If I apply a style to something like a paragraph, and then apply some font-related format to something inside the paragraph, it overrides the entire style, instead of adding to it.

For instance {{<p style="font-size: 10px">This has <i>italics</i> in it</p> }}should display everything in very tiny type, with one word italicized. Instead, the italicized word reverts to the normal text size. This happens any time formatting is applied to a portion of an element, including by a "style" attribute, or a CSS class name. This is not how web browsers work.