vitest >=5.0.0 changed the default of `HTML reporter`. HTML reporter outputDir option default value is .vitest; the report entry is written to .vitest/index.html. Silent: code written for older versions still runs, without an error.

const fs = require("fs");
const path = require("path");
const cp = require("child_process");
const dir = "/w/htmlreport";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
fs.writeFileSync(path.join(dir, "package.json"), "{}\n");
fs.writeFileSync(path.join(dir, "basic.test.mjs"), [
  "import { test, expect } from 'vitest'",
  "test('ok', () => { expect(1 + 1).toBe(2) })",
].join("\n") + "\n");
const r = cp.spawnSync(process.execPath, ["/w/node_modules/vitest/vitest.mjs", "run", "--reporter=html"], {
  cwd: dir,
  encoding: "utf8",
  env: { ...process.env, NO_COLOR: "1", CI: "1" },
  timeout: 70000,
});
console.log((r.stdout || "") + (r.stderr || "") + (r.error ? String(r.error) : ""));
const newEntry = fs.existsSync(path.join(dir, ".vitest", "index.html"));
const oldEntry = fs.existsSync(path.join(dir, "html", "index.html"));
console.log("exit " + r.status + ", .vitest/index.html " + newEntry + ", html/index.html " + oldEntry);
if (newEntry && !oldEntry) {
  console.log("PASS: --reporter=html wrote its report entry to .vitest/index.html");
  process.exit(0);
}
if (oldEntry && !newEntry) {
  console.error("FAIL: --reporter=html wrote its report entry to html/index.html, not .vitest/index.html");
  process.exit(1);
}
console.error("ERROR: no single html report entry found (exit " + r.status + ")");
process.exit(2);

const fs = require("fs");
const path = require("path");
const cp = require("child_process");
const dir = "/w/htmlreport";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
fs.writeFileSync(path.join(dir, "package.json"), "{}\n");
fs.writeFileSync(path.join(dir, "basic.test.mjs"), [
  "import { test, expect } from 'vitest'",
  "test('ok', () => { expect(1 + 1).toBe(2) })",
].join("\n") + "\n");
const r = cp.spawnSync(process.execPath, ["/w/node_modules/vitest/vitest.mjs", "run", "--reporter=html"], {
  cwd: dir,
  encoding: "utf8",
  env: { ...process.env, NO_COLOR: "1", CI: "1" },
  timeout: 70000,
});
console.log((r.stdout || "") + (r.stderr || "") + (r.error ? String(r.error) : ""));
const newEntry = fs.existsSync(path.join(dir, ".vitest", "index.html"));
const oldEntry = fs.existsSync(path.join(dir, "html", "index.html"));
console.log("exit " + r.status + ", .vitest/index.html " + newEntry + ", html/index.html " + oldEntry);
if (newEntry && !oldEntry) {
  console.log("PASS: --reporter=html wrote its report entry to .vitest/index.html");
  process.exit(0);
}
if (oldEntry && !newEntry) {
  console.error("FAIL: --reporter=html wrote its report entry to html/index.html, not .vitest/index.html");
  process.exit(1);
}
console.error("ERROR: no single html report entry found (exit " + r.status + ")");
process.exit(2);