css/css-conditional/at-supports-044.html

<!DOCTYPE html>
<html lang="en">
<meta charset="UTF-8">

<title>CSS Conditional Test: Custom Properties in @supports</title>
<meta name="assert"
      content="Test passes if custom properties are supported and correctly detected in @supports,
               or unsupported and correctly detected in @supports.">
<link rel="help" href="https://www.w3.org/TR/css3-conditional/#at-supports">
<link rel="help" href="https://www.w3.org/TR/css-variables-1/">
<link rel="author" title="Elika J. Etemad" href="http://fantasai.inkedblade.net/contact">
<link rel="match" href="at-supports-001-ref.html">

<style>
  div {
    background: green;
    height: 20px;
    width: 100px;
  }

  body {
    --badcolor: red;
    --goodcolor: green;
  }
  div {
    background: var(--badcolor);
  }
  @supports (--foo: whatever) {
    .test1 { background: green; }
  }

  @supports (--foo: whatever !important) {
    .test2 { background: var(--goodcolor); }
  }

  .test3 { background: green; }
  @supports (--foo: whatever !bogus) {
    .test3 { background: red; }
  }

  @supports (color: var(--anything) invalid-value) {
    .test4 { background: green; }
  }

  .test5 { background: red; }
  @supports not (--goodcolor: green) {
    .test5 { background: green; }
  }
  .test5 { background: var(--goodcolor) }
</style>

<p>Test passes if there is a <strong>filled green square</strong> and <strong>no red</strong>.</p>

<div class="test1"></div>
<div class="test2"></div>
<div class="test3"></div>
<div class="test4"></div>
<div class="test5"></div>