Drop postcss

[SylvainCorlay]

This makes widgetsnbextension and html-manager use the raw CSS from @jupyter-widgets/controls instead of the version processed with postcss and cssnext.

• cssnext was substituting CSS variables with their values.
• with this change, the CSS variables that are defined to polyfill for JupyterLab in the classic notebook become visible and usable by widgets. This will allow custom widget libraries to handle lab, classic, and embedded contexts in a more homogeneous way.
• cssnext is deprecated.

Note: that IE does not support CSS variables, but it is not a supported browser for the classic notebook at the moment.

[SylvainCorlay]

@jasongrout as per our discussion.

[maartenbreddels]

I see a lot of red 👍 (as in lines removed).
To summarize, instead of substituting the variables, we now include them in the controls package? Which means the become visible in the notebook once the controls package is loaded?

[SylvainCorlay]

To summarize, instead of substituting the variables, we now include them in the controls package? Which means the become visible in the notebook once the controls package is loaded?

widgets.css includes the variables. (all it does is @import labvariables.css and @import widgtets-base.css).

[SylvainCorlay]

@maartenbreddels @jasongrout how about we press the green button?

[jasongrout]

Reviewing this is next on my list, after a short break :)

[jasongrout]

Thanks. I'll note that we still build widgets.built.css so people can still use it from the @jupyter-widgets/controls package if they wish.