User-agent stylesheet

The default CSS bundled with the browser that applies before any author or user style. Sets the baseline rendering of every HTML element; differs subtly across Chromium, WebKit, and Gecko.

Every browser ships a UA stylesheet that gives <button> its default border, <h1> its default size, <ul> its bullet, and so on. Cross-engine differences in these defaults are the reason projects ship a CSS reset (Eric Meyer’s reset, normalize.css) or a baseline layer (Tailwind’s preflight). Authors can read the UA stylesheet via developer tools to understand any unfamiliar inherited value.