The appearance of a custom view is, by definition, custom, and so not much can be said about this view type.
A standard row guide can appear to the left of the user-defined HTML code if the row-guided flag of the view is set to true, otherwise the custom HTML occupies the entire space of the report page (to the right of the criteria filter, of course).
As described on the Custom Views page, the report page will render the "Header" code once at the top, then the "Body" code below that - either a single time for the selected query result if the view is row-guided, or otherwise repeated for each query result - followed by the "Footer" HTML code, and makes the supplied CSS and JS available to that block.
This view type can only assume the "Report Page" designation, though the same view can be achieved as a dashboard widget using Custom Widgets.