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how to add effect to open the submenu to have a transition fade effect? (cssmenu)

The CSS menu does not have any 'open effects'. But it does support transitions.

Alternatively, you can use the drop down menu which does support animated sub menus.

In this case, in the menu I (dropdawn), can I have an effect on the submenu and the main menu as well?

I didn't find the same effects as in the css menu.

These menus serve different purposes, each with its own unique functionality. If they all had the same functionality, there would be no need for separate tools.

If you are going to apply css code, how should you add a fade when opening the submenu to the menu (cssmenu)?

I don't think that will work for the CSS menu because the position of the sub menu changes on hover.