swc/crates/swc_css_parser/tests/fixture/value/custom-property/input.css

:root {
    --:value;

    --important:value!important;
    --important1: value!important;
    --important2: value !important;
    --important3:value !important;
    --important4: calc(1)!important;

    --empty: ;
    --empty2: /**/;
    --empty3: !important;
    --empty4:/**/ !important;
    --empty5:/* 1 */ /* 2 */;

    --no-whitespace:ident;
    --number: 1;
    --unit: 100vw;
    --color: #06c;

    --function: calc(1 + 1);
    --variable: var(--unit);

    --string: 'single quoted string';
    --string: "double quoted string";

    --square-block: [1, 2, 3];
    --square-block1: [];
    --square-block2:[];
    --round-block: (1, 2, 3);
    --round-block1: ();
    --round-block2:();
    --bracket-block: {1, 2, 3};
    --bracket-block1: {};
    --bracket-block2:{};


    --JSON: [1, "2", {"three": {"a":1}}, [4]];
    --javascript: function(rule) { console.log(rule) };

    --CDO: <!--;
    --CDC: -->;

    --complex-balanced:{[({()})()()[({})]]}[{()}]([]);
    --fake-important:{!important};
    --semicolon-not-top-level: (;);
    --delim-not-top-level: (!);
}

:root{--a:1}
:root {--foo: }
:root {
    --foo: 
}