A real Custom Properties polyfill for IE11.
Used on about 8000 Live Websites

• chaining --bar:var(--foo)
• fallback var(--color, blue)
• :focus, :target, :hover
• handle dynamic added html-content
• handle dynamic 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
• SVG-support
• media-queries (redraw on media-changes)
• transform relative to absolute urls
• under 3k (min+gzip) and dependency-free

https://rawcdn.githack.com/nuxodin/ie11CustomProperties/132e9517197ba1d9ca71102dcc7e3d76fffb5835/test.html

You only want IE11 to load the polyfill, use this snipped 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>

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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 Element and get its 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 it

...is always little highter, cause each selector gets an additional class-selector eg. #header results in #header.iecp_u44