Problem can be observed in both QML Text and QTextEdit. So I suppose it is a problem in Qt support for rich text as a whole.

So first of all, <margin-*> is a supported property:

https://doc.qt.io/qt-6/richtext-html-subset.html#css-properties

also <span>

https://doc.qt.io/qt-6/richtext-html-subset.html#supported-tags

And actually using <span> with most of other CSS properties work just fine - except <margin-*>. 2 reproducers attached: one for QML and one for QWidget. Build with 6.7.2 and run them, then one can see that margin is missing. The same rich text should look like as what a browser shows:

But actually margin is not there. QTextEdit:

Quick:

Actually, one can see that all other properties (color, font, etc.) work correctly. But not <margin-*>.

Also, the problem seems to be related to combination of <span> and <margin-*>. For example, using <maring-*> like <p style='margin-left: 200px;'> is totally fine.