I know that web dialogs functionality is deprecated in the new unified interface. But we still need it. I have found out that there is a "Xrm.Internal.openLegacyWebDialog" function that allows to open html pages in native CRM window like "Xrm.Internal.openDialog" function in classic interface. But "closeWindow" function doesn't work and also I didn't find a way how to change zindex of the dialog window. Early I was able to do it by passing zindex parameter into "Mscrm.InlineDialogUtility.createInlineDialog" function.
Thanks in advance for any help with it!