CSS custom properties polyfill for ie11

Have a look at this (my) Custom-Properties-Polyfill:
https://github.com/nuxodin/ie11CustomProperties

How it works

The script makes use of the fact that IE has minimal custom properties support where properties can be defined and read out with the cascade in mind.
.myEl {-ie-test:'aaa'} // only one dash allowed "-"
then read it in javascript:
getComputedStyle( querySelector('.myEl') )['-ie-test']

From the README:

Features

• handles dynamic added html-content
• handles dynamic added , -elements
• chaining --bar:var(--foo)
• fallback var(--color, blue)
• :focus, :target, :hover
• js-integration:
  • style.setProperty('--x','y')
  • style.getPropertyValue('--x')
  • getComputedStyle(el).getPropertyValue('--inherited')
• Inline styles: <div ie-style="--color:blue"...
• cascade works
• inheritance works
• under 3k (min+gzip) and dependency-free

Demo:

https://rawcdn.githack.com/nuxodin/ie11CustomProperties/b851ec2b6b8e336a78857b570d9c12a8526c9a91/test.html

You didn't mention how you're bundling your JavaScript, but yes, it's possible. For example, PostCSS has a plugin, which polyfills this feature.

The usage depends on how you're bundling your script files. With Webpack, for example, you'd define this plugin in your postcss config or import it as a plugin under your webpack config:

// webpack.config.js:
module.exports = {
  module: {
    rules: [
        {
            test: /\.css$/,
            use: ["style-loader", "css-loader", "postcss-loader"]
        }
    ]
  }
}

// postcss.config.js
module.exports = {
  plugins: [
    require('postcss-custom-properties'),
    require('autoprefixer'),
    // any other PostCSS plugins
  ]
}

The plugin also has an example for programmatic usage (as a separate node script):

// dependencies
var fs = require('fs')
var postcss = require('postcss')
var customProperties = require('postcss-custom-properties')

// css to be processed
var css = fs.readFileSync('input.css', 'utf8')

// process css using postcss-custom-properties
var output = postcss()
  .use(customProperties())
  .process(css)
  .css