Example of how to run vscode's built-in CSS validations from the command-line. Use `node validate-css.mjs *.css`.

validate-css.mjs

import fs from "fs";
import url from "url";

import { getCSSLanguageService } from "vscode-css-languageservice";
import { TextDocument } from "vscode-languageserver-textdocument";

/** Human-readable names for the potential diagnostic severities returned. */
const severities = { 1: "ERR", 2: "WARN", 3: "INFO", 4: "HINT" };

/** Format a position in a given file. */
const formatPosition = (path, position) =>
    `${path}:${position.line + 1}:${position.character + 1}`;

/** Format a single diagnostic in a human-friendly fashion. */
const formatDiagnostic = (diagnostic) =>
    `${formatPosition(diagnostic.path, diagnostic.range.start)} [${severities[diagnostic.severity]}] ${diagnostic.message} ${diagnostic.source}(${diagnostic.code})`;

const service = getCSSLanguageService();

// Walk through all target files, generating diagnostics for each.
const diagnostics = process.argv.slice(2).flatMap(path => {
    const lintUrl = url.pathToFileURL(path);
    const lintText = fs.readFileSync(path, "utf8");
    const lintDocument = TextDocument.create(lintUrl.toString(), "css", 0, lintText);
    const validation = service.doValidation(lintDocument, service.parseStylesheet(lintDocument));
    return validation.map(diagnostic => ({...diagnostic, path}));
});

// Print out the diagnostics, if any.
if (diagnostics.length > 0) {
    console.error(diagnostics.map(formatDiagnostic).join("\n"));
    process.exit(1);
}

You'll want to npm install --save-dev vscode-css-languageservice.