Custom Properties polyfill for IE11.
npm install ie11-custom-properties--bar:var(--foo)
var(--color, blue)
, -elements
style.setProperty('--x','y')
style.getPropertyValue('--x')
getComputedStyle(el).getPropertyValue('--inherited')
CSS.registerProperty({name:'--red', inherit:false, initialValue:'#e33'}) (of course not animatable)
- 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
Usage
You only want IE11 to load the polyfill, use this snippet in the head of your html file, it just works:
``html
`
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:
1. given the CSS
`css
header { --myColor:red; }
main { --myColor:green; }
li { color:var(--myColor); }
`
2. rewritten CSS
`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
`js
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
`css
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 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`
Tests
See the tests
PRs welcome