CSS font-family to new FontAwesome version incorrect

[scimax]

Describe the issue

Hello,
I'm facing a problem with the FontAwesome icons, as I first mentioned at matplotlib/ipympl#274. But the issue is not related to ipympl but rather to ipywidgets. So, first of all, this is what the problem looks like when using the fontawesome icons:

import ipywidgets.widgets as widgets
widgets.Button(description="Hello", icon='check')

Output:

After reading through past issues and the history of the developement I learned that there was a change in the FontAwesome version a few month ago, e.g. mentioned here #2848 by @martinRenou , but I couldn't track down the origin by myself. With the help from @Chaoste, we found the origin in the CSS class .fa. In the browser inspector it reads as follows:

.fa {
    display: inline-block;
    font: normal normal normal 14px/1 FontAwesome;
    font-size: inherit;
    text-rendering: auto;
    -webkit-font-smoothing: antialiased;
}

but three style sections below (superseded by the one above) the font-family had a different name:

.fa, .far, .fas {
    font-family: "Font Awesome 5 Free";
}

When we changed it in the top class it didn't immediately change, but with font: 900 normal normal 14px/1 "Font Awesome 5 Free"; the icon appeared:

According to this stack post it should be "Font Awesome 5 Free" isn't it? Or is a reference from "FontAwesome" to "Font Awesome 5 Free" somehow missing? I'm a little bit confused as I still don't know where to fix it permanently. Any help would be appreciated.

Versions

Output of

PS > python -c "import sys; print('\n',sys.version); import ipympl; print('ipympl version:', ipympl.__version__)" ; jupyter --version ; jupyter nbextension list ; jupyter labextension list

 3.6.10 (default, Mar  5 2020, 10:17:47) [MSC v.1900 64 bit (AMD64)]
ipympl version: 0.5.8
jupyter core     : 4.6.3
jupyter-notebook : 6.1.4
qtconsole        : not installed
ipython          : 7.16.1
ipykernel        : 5.3.4
jupyter client   : 6.1.7
jupyter lab      : 2.2.2
nbconvert        : 6.0.7
ipywidgets       : 7.5.1
nbformat         : 5.0.8
traitlets        : 4.3.3
Known nbextensions:
  config dir: C:\Users\maxke\.jupyter\nbconfig
    notebook section
      jupyter-js-widgets/extension enabled
      - Validating: ok
  config dir: C:\Users\maxke\.conda\envs\jlab\etc\jupyter\nbconfig
    notebook section
      jupyter-matplotlib/extension enabled
      - Validating: ok
      nbdime/index disabled
      jupyter-js-widgets/extension enabled
      - Validating: ok
JupyterLab v2.2.2
Known labextensions:
   app dir: C:\Users\maxke\.conda\envs\jlab\share\jupyter\lab
        @jupyter-widgets/jupyterlab-manager v2.0.0 enabled  ok
        @jupyterlab/debugger v0.2.0 enabled  ok
        @jupyterlab/toc v4.0.0 enabled  ok
        @krassowski/jupyterlab_go_to_definition v1.0.0 enabled  ok
        @lckr/jupyterlab_variableinspector v0.5.1 enabled  ok
        jupyter-matplotlib v0.7.4 enabled  ok
        nbdime-jupyterlab v2.0.0 enabled  ok
        spreadsheet-editor v0.4.0 enabled  ok

[scimax]

I have no clue where it came from. But after creating a completely new conda environment it works (still the same packages as before).