Headings (<h1>, <h2>) and so on have to descend without skipping, i.e. after an <h2>, the next one shouldn’t be an <h4>.

The package names in the CSS reference guide, for example javafx.stage and javafx.scene, are represented by tables:

    <h2><a id="stage">Stage</a></h2>
    <table class="package" width="100%">
        <tbody>
        <tr>
            <td>javafx.stage</td>
        </tr>
        </tbody>
    </table>
    <h4><a id="popupwindow">Group</a></h4>

javafx.stage should just be an <h3>:

    <h2><a id="stage">Stage</a></h2>
    <h3 class="package">javafx.stage</h3>
    <h4><a id="popupwindow">Group</a></h4>

Actually, the stylesheet defines the .package style, which can be applied to an h3 to make it look exactly like what it looks like with tables.

I think this is an easy search-and-replace to do. There are 11 instances of <table class="package" on the page, which correspond to the 11 packages documented.

The FXML reference does *not* have this issue.