A driver of your own

A test may drive a browser of its own, with the cookie of a session for its user set into it (app.session). These are then the test's own:

The browser. It is the driver's, fetched as its own documents say: tent add @playwright/test, then npx playwright install chromium in the folder of the project fetches the Chromium that this version of Playwright drives, and npx playwright install chromium firefox webkit the three engines. app.open uses the builds that the engine pins, and never these.
The address. A test app answers on no port: every request of app.open and app.browser reaches it in the program of the test. A driver reaches an app over a port instead: one that the test starts with tent start --mode test --seed --port 0, which names its address on the standard error and is ended by its number (command.start), or one that a program of the test opens with createEngine({ port: 0 }) (topic.engine), whose app.session makes the cookie. Such a program fakes a model by makers.providers, a provider of its own under the name of the model's provider (under "A faked model", above). A walk of an app that a person would take signs in through the app's own sign-in screen.
A microphone. app.open gives a page none. A test of useMicrophone starts its own Chromium with the browser's own flags for a recording that is no device, --use-fake-ui-for-media-stream, --use-fake-device-for-media-stream and --use-file-for-fake-audio-capture=<a .wav file>, so the page hears that file. A Firefox of its own takes the preferences media.navigator.streams.fake and media.navigator.permission.disabled, both true, and hears a tone in the place of a file. Playwright's WebKit on Windows opens no microphone, so a step that speaks is walked in the other two there.
Motion. A picture of page.picture() is taken as the page stands at that moment: a step waits for what it set off by fact, and for no transition, animation or view transition to end. A driver of its own asks for less motion with reducedMotion: 'reduce' of the browser's context in Playwright, so a page whose CSS keeps its motion inside @media (prefers-reduced-motion: no-preference) is pictured at rest.

Such a test finds a thing as a person does: by what it is and by its words, and never by a class or an id of the page, which are the page code's own and may change.

To find	By	With Playwright
What is pressed	Its role, `button` or `link`, and its words or its `aria-label`	`page.getByRole('button', { name: 'Mark paid' })`
A field	Its label: the `<label>` around it or named by `for`, or the label that `useField` gives from its type	`page.getByLabel('Total')`
A form	The `aria-label` of the `Form`	`page.getByRole('form', { name: 'New order' })`
A heading, a text	Its words	`page.getByRole('heading', { name: 'Orders' })`, `page.getByText('Ada Lovelace')`
The words of a failure	The field they belong to: they describe it	`page.getByLabel('Total')` is described by them: `await expect(page.getByLabel('Total')).toHaveAccessibleDescription('This is needed.')`. An error that names no field is an `alert` after the last field of its form
A field in error	Its state	`aria-invalid` is `true` on it

expect in the table is of the package @playwright/test, which a project installs for it: tent add @playwright/test. The driver alone, playwright-core, holds every page.getBy... of the table and no expect. With it a test reads the words of a failure itself, through the field that they describe, and holds them with node:assert:

TypeScript

const total = page.getByLabel('Total')
const words = await total.evaluate(
  (field) => document.getElementById(field.getAttribute('aria-describedby') ?? '')?.textContent,
)
assert.equal(words, 'This is needed.')
assert.equal(await total.getAttribute('aria-invalid'), 'true')

A page is sent as HTML and comes alive when its code has loaded. A press before that is lost, so a test waits for the mark first: await page.locator('#tent[data-t-alive]').waitFor() (topic.screens). No header and no field of a request of the web names one.