A real Custom Properties polyfill for Internet Explorer 11.
Used on about 11'000 Live Websites

See the demo!

New in v4.0.0: Very fast now

• chaining --bar:var(--foo)
• fallback var(--color, blue)
• :focus, :target, :hover
• handle dynamically added html-content
• handle dynamically added <style>, <link>-elements
• js-integration:
  • style.setProperty('--x','y')
  • style.getPropertyValue('--x')
  • getComputedStyle(el).getPropertyValue('--inherited')
• Houdini’s new API: CSS.registerProperty({name:'--red', inherit:false, initialValue:'#e33'}) (of course not animatable)
• style-attributes <div ie-style="--foo:bar"...
• cascade works
• inheritance works
• !important on setters and getters
• inherit, initial, unset and revert keyword for variables
• SVG support
• media-queries (redraw on media-changes)
• transform relative to absolute urls
• under 4k (min+gzip) and dependency-free

You only want IE11 to load the polyfill, use this snippet in the head of your html file, it then just works:

<script>window.MSInputMethodContext && document.documentMode && document.write('<script src="yourJsPath/ie11CustomProperties.js"><\x2fscript>');</script>

• Add a ⭐️
• Vote for this solution at stackoverflow
  https://stackoverflow.com/a/57000437/4865307 and
  https://stackoverflow.com/a/57000620/4865307
• Test, report bugs and send pull requests.
• Tweet about if you like it.

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. This is not possible with properties starting with double dashes. .myEl {-ie-test:'aaa'} // only one dash allowed! "-" then you can read it in IE with javascript: getComputedStyle( querySelector('.myEl') )['-ie-test'] In the raw CSS, it replaces for example --foo with -ie-foo. It searches for all rules containing variable getters and setter, remembers the affected selectors so future affected Elements can be found in a mutation observer. Each affected Element gets a uniq class-attribute and its own style-sheet to draw the Element. These are the steps that the script does:

1. given the CSS

header { --myColor:red; }
main { --myColor:green; }
li { color:var(--myColor); }

2. rewritten CSS

header { -ie-myColor:red; }
main { -ie-myColor:green; }
li { -ieHasVar-color:var(-ie-myColor); }

3. find all affected Elements and get their property-values

querySelectorAll('li').forEach(function(){
    var color = getComputedStyle(this).getPropertyValue('--myColor');
    // getPropertyValue is extended to handle custom properties
    // draw_the_Element()
})

3. draw Elements, this leads in separate rules for each Element

li.iecp-u1 { color:red; }
li.iecp-u2 { color:red; }
li.iecp-u3 { color:green; }
li.iecp-u4 { color:green; }

There is no way to get the raw content of style-attributes in IE11. Use <div style="--color:blue" ie-style="--color:blue"> for this.

...is always little higher if vars are not served by root, because each selector gets an additional class-selector eg. #header results in #header.iecp_u44

See the tests
PRs welcome