CSS Variables Polyfill for IE11

The most complete "Custom Properties polyfill" for Internet Explorer 11.
Used on about 30'000 Live Websites

Discontinued

The IE11 market share is steadily decreasing. I personally have no need to support IE11 anymore.
That's why I have decided to stop working on this project.

Features

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 (see limitations)
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

Usage

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"><\/script>');</script>

Help wanted!

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.

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:

given the CSS

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

rewritten CSS

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

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()
})

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; }

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

@import

vars in @import is not supported at the moment

!important

There are problems with !important. I need a minimal, reproducible testcase on https://jsbin.com/

Tests

See the tests
PRs welcome

cicec/ie11CustomProperties