node_modules/csso/lib/replace/AttributeSelector.js

// Can unquote attribute detection
// Adopted implementation of Mathias Bynens
// https://github.com/mathiasbynens/mothereff.in/blob/master/unquoted-attributes/eff.js
var escapesRx = /\\([0-9A-Fa-f]{1,6})(\r\n|[ \t\n\f\r])?|\\./g;
var blockUnquoteRx = /^(-?\d|--)|[\u0000-\u002c\u002e\u002f\u003A-\u0040\u005B-\u005E\u0060\u007B-\u009f]/;

function canUnquote(value) {
    if (value === '' || value === '-') {
        return;
    }

    // Escapes are valid, so replace them with a valid non-empty string
    value = value.replace(escapesRx, 'a');

    return !blockUnquoteRx.test(value);
}

module.exports = function(node) {
    var attrValue = node.value;

    if (!attrValue || attrValue.type !== 'String') {
        return;
    }

    var unquotedValue = attrValue.value.replace(/^(.)(.*)\1$/, '$2');
    if (canUnquote(unquotedValue)) {
        node.value = {
            type: 'Identifier',
            loc: attrValue.loc,
            name: unquotedValue
        };
    }
};