Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

I'm not sure the exact purpose of CSS variables when you are working in a language that has variables. It seems like this is a 2nd way to do things we can already do differently, except now without any compiler help for types and typos.

I'm not against doing this particularly, but so far I have not seen any cases that definitively show that this is a good thing to have. I prefer to use GitHub for bugs, not feature requests, so I will close.

@evancz This isn´t a feature request, this is a bug report. Though I might be too polite to describe it as such.

This library is missing a feature that is a part of the CSS standard. I´m not familiar with the mindset behind the library, but my initial expectation is that it supports the standard that exists.

@evancz I found this bug while trying to pass an --indent variable to specify how many "spaces" a piece of HTML has to be indented. The exact amount of pixels is calculated in the CSS using a calc(). But I don't wont to make this calculation in pixels in my Elm code, because I prefer to have all the style details in my CSS file, so I don't see this as 2nd way of doing the same thing.

I believe that in the virtual dom source at line 509 (https://github.com/elm-lang/virtual-dom/blob/master/src/Elm/Kernel/VirtualDom.js#L509), if

domNodeStyle[key] = styles[key]

was replaced with

domNodeStyle.setProperty(key, styles[key])

then custom variables would work.

CSS custom properites are designed to be used in JavaScript (see https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables#Values_in_JavaScript) and I think it would be really useful if elm allowed custom properties to be set.

Many thanks for the great libary :)