css/css-variables/variable-supports-09.html

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<!DOCTYPE html>
<title>CSS Test: Test a failing non-custom property declaration in an @supports rule where the property value contains a syntactically invalid variable reference due to having a '!' token at the top level of its fallback.</title>
<link rel="author" title="Cameron McCormack" href="mailto:cam@mcc.id.au">
<link rel="help" href="http://www.w3.org/TR/css-variables-1/#using-variables">
<link rel="match" href="support/color-green-ref.html">
<style>
body { color: red; }
@supports (color: var(--a)) and (not (color: var(--a,!))) {
  p { color: green; }
}
</style>
<p>This text must be green.</p>