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    4.1.0 • Public • Published

    CSS Variables Polyfill for IE11

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

    See the demo!


    • 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 just works:

    <script>window.MSInputMethodContext && document.documentMode && document.write('<script src="https://cdn.jsdelivr.net/gh/nuxodin/ie11CustomProperties@4.1.0/ie11CustomProperties.min.js"><\x2fscript>');</script> 

    Help wanted!

    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. 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); }
    1. rewritten CSS
    header { -ie-myColor:red; }
    main { -ie-myColor:green; }
    li { -ieHasVar-color:var(-ie-myColor); }
    1. find all affected Elements and get their property-values
        var color = getComputedStyle(this).getPropertyValue('--myColor');
        // getPropertyValue is extended to handle custom properties
        // draw_the_Element()
    1. 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; }


    Small limitations

    Styles in element-attributes

    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.

    Specificity for properties containing "var()"

    ...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


    npm i ie11-custom-properties

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