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                html {
  color: red;
  /* ignored as *invalid syntax* by all browsers */
  /* - old browsers: red */
  /* - new browsers: red */
  color: not a valid color; 
  color: var(not a valid variable name); 
  /* ignored as *invalid syntax* by browsers without var support */
  /* valid syntax, but invalid *values* in modern browsers */
  /* - old browsers: red */
  /* - new browsers: unset (black) */
  --invalid-value: not a valid color value;
  color: var(--undefined-variable);
  color: var(--invalid-value);

/* browser support flag */
html::before {
  content: 'your browser does NOT support CSS Variables…';
  background: red;
  display: block;
  padding: 1em;

@supports (--var: 'any') {
  html::before {
    content: 'your browser DOES support CSS Variables…';
    background: lightgreen;