TINMYMCE delete <link href="../portal_skins/custom/ITProva.css" rel="stylesheet" type="text/css" />

I have this problem:
i have a simple html menu on sx portlet

but when save with source code, the string " is delete :sob::sob: can help me please ?

look at the HTML filtering settings of Plone. Of Plone will delete <link> and <script> tags by default and access to the HTML source functionality in TinyMCE should be forbidden be default as well. Content editors should never ever be able to work on HTML directly.

See https://training.plone.org/5/mastering-plone/configuring_customizing.html
Note esp. section 10.4.2 about portal_skins

Thank you
is a company intranet and managed by me.
I have to migrate from 4.3 to 5.1. and there are a few pages with CSS style.
for test i disabled html filtering, but not view css style

on 4.3 it works properly

I am not sure if the correct word is 'properly'.

To add CSS and javascript in the editor is not a very good idea .

Is there ay reason why you can not add it for example in the theme ?