How to feature detect for CSS custom properties? [duplicate]

Question

Is there any way to detect whether a browser supports CSS custom properties (eg. color: var(--primary))?

I need to write a script that behaves slightly differently in browsers that don't support custom properties, but I can't find any documented Modernizr tests, nor any information about the Javascript interface for accessing custom properties.

Would be grateful for any suggestions!

Answer 1

You can reliably use CSS.supports() in JavaScript or @supports in CSS to detect support for custom properties. Every version of every browser that supports custom properties also supports this method of feature detection.

if (window.CSS && CSS.supports('color', 'var(--primary)')) {
  document.body.innerHTML = 'Supported';
}

@supports (color: var(--primary)) {
  body {
    background-color: green;
  }
}

Notice that this does not require you to declare the --primary custom property, as by definition every property whose name begins with -- is considered valid for the purposes of parsing property declarations.

Answer 2

I'll leave this as an example of how to approach this sort of thing when you don't have an alternative, but BoltClock's answer is the way to go in this case.

---

I expect you could define one and then check whether its value comes through to getComputedStyle. For instance, this shows Supports custom props? true on Chrome (which supports custom CSS properties) but Supports custom props? false on IE11 (which doesn't):

function areCSSVarsSupported() {
  var d = document.createElement('div');
  d.id = "test";
  document.body.appendChild(d);
  var pos = getComputedStyle(d).position;
  document.body.removeChild(d);
  return pos === "fixed";
}
console.log("Supports custom props? " + areCSSVarsSupported());

:root{
  --test-vars: fixed;
}

#test {
  position: var(--test-vars);
}