I have a single spa application with a root-config, a styleguide, and a couple of react applications. All of these application are generated using the yarn create single-spa command.
I tried creating a kit from the font-awesome website and adding it to the root-config index.ejs inside the head tag.
Then I tried adding <i className='fa-brands fa-facebook-f' /> in one of my MFEs.
The loaded index.html in my browser include the tag code. But it's dimensions are 0x0.
You are missing some styles.
When you generate your kit, the font awesome website shares a snippet of the script tag for your kit.
You need to place it in the head section of your root-config.
But that's not all. You also need to add some font-faces.
The same page (with the snippet) also has a link to download example html file.
If you check that file it has extra style tags.
Once you add those to your root-config, the icons will start showing up.
This issue is not single-spa, it's the confusing documentation for font-awesome.