User-Agent
// Definition
An HTTP request header that identifies the client software — browser name, version, rendering engine, operating system, and device type. Sent automatically by browsers; can be set to any value by non-browser clients and automation tools. Servers may respond differently based on the UA string (mobile redirects, bot-blocking, browser-specific assets). QA considerations: headless Playwright and Cypress send a recognisable headless UA that can trigger bot-detection or return a stripped response — set a realistic UA string when testing UA-gated paths. Parsing UA strings to infer device capabilities is fragile; prefer server-side feature detection. The Chrome UA Reduction initiative is progressively freezing minor version numbers to improve privacy, which can break UA-parsing-based analytics.
// Related terms
Cross-Browser Testing
Verifying a web application works correctly across different browsers (Chromium, Firefox, WebKit) and versions. Modern frameworks like Playwright cover all three rendering engines from one suite.
HTTP Header
A key-value metadata field attached to an HTTP request or response, transmitted before the body. Request headers describe the client and request context (User-Agent, Accept, Content-Type, Authorization, Cookie); response headers describe the server's response and instruct the client (Content-Type, Set-Cookie, Cache-Control, CORS access-control headers, security headers). Header names are case-insensitive. QA testing checklist: assert Content-Type matches the body format; verify security headers are present on responses (HSTS, CSP, X-Frame-Options, X-Content-Type-Options); confirm sensitive request headers (Authorization, Cookie) are not logged or exposed in error responses; verify CORS headers permit only expected origins.
Headless Browser
A real browser running without a visible UI. Used in CI for speed and reproducibility — same rendering engine as a normal browser, with no display required.