node_modules/csso/lib/replace/property/border.js

function removeItemAndRedundantWhiteSpace(list, item) {
    var prev = item.prev;
    var next = item.next;

    if (next !== null) {
        if (next.data.type === 'WhiteSpace' && (prev === null || prev.data.type === 'WhiteSpace')) {
            list.remove(next);
        }
    } else if (prev !== null && prev.data.type === 'WhiteSpace') {
        list.remove(prev);
    }

    list.remove(item);
}

module.exports = function compressBorder(node) {
    node.children.each(function(node, item, list) {
        if (node.type === 'Identifier' && node.name.toLowerCase() === 'none') {
            if (list.head === list.tail) {
                // replace `none` for zero when `none` is a single term
                item.data = {
                    type: 'Number',
                    loc: node.loc,
                    value: '0'
                };
            } else {
                removeItemAndRedundantWhiteSpace(list, item);
            }
        }
    });
};