var postcss = require('postcss');
var fallback = require('postcss-fallback');
var fs = require('fs');
var mycss = fs.readFileSync('input.css', 'utf8');
// Process your CSS with postcss-fallback
var output = postcss([
		fallback(/*options*/)
	])
	.process(mycss)
	.css;
console.log(output);

.foo {
	display: fallback(flex, inline-block);
	width: fallback(45vh, 450px);
	background-color: fallback(rgba(0, 0, 0, 0.5), #555555);
	foo: fallback(bar, baz, qux, corge);
}

.foo {
	display: inline-block;
	display: flex;
	width: 450px;
	width: 45vh;
	background-color: #555555;
	background-color: rgba(0, 0, 0, 0.5);
	foo: corge;
	foo: qux;
	foo: baz;
	foo: bar;
}