I don’t know if this helps, but I tend to create my text using the visual editor in a draft WP page, get it how I want it to look and then click the text tab and cut and paste the code into the text widget. Seems to work.
Wordpress’ text widget supports HTML and it does not do sanitization.
If your HTML is valid, it will be displayed correctly, otherwise it will break the layout of the site (more or less).