css/css-variables/variable-declaration-11.html

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<!DOCTYPE html>
<title>CSS Test: Test declaring a variable with invalid syntax due to a variable reference 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/#syntax">
<link rel="match" href="support/color-green-ref.html">
<style>
p {
  color: red;
  --a: green;
  --b: crimson;
  --a: var(--b,!);
  color: var(--a);
}
</style>
<p>This text must be green.</p>