Web Browser (User Agent) Built-In CSS

Every web browser has a built-in (User Agent) style sheet for rendering default HTML elements. Here are the locations for those UA style sheets:

• W3C & WHATWG
  • HTML 4.01
  • HTML5
  • HTML5 Living Standard
  • CSS 1
  • CSS 2
  • CSS 2.2
• Firefox
  • Mercurial, at mozilla-central/layout/style/res/html.css (or look at dxr.mozilla.org)
  • GitHub, at gecko-dev/layout/style/res/html.css
  • In Firefox, open resource://gre-resources/ & then select html.css
• Safari, at trunk/Source/WebCore/css/html.css
• Chrome & Opera, at chromium/chromium/blink/master/Source/core/css/html.css
• Microsoft
  • IE 6–9 (Trident) (Note that the bottom of this page contains links to user-agent style sheets for Edge, Chrome, Firefox, & Safari)
  • Edge 13
• Opera (pre-Blink)

UA style sheets from 2014 can be found on GitHub at sw4/revert.css.

HTML5 Boilerplate is an attempt to solve any inconsistencies (which there aren’t many of anymore).

Original pointers from Stack Overflow & Jens Oliver Meiert.

Updated 2017-10-30 to fix broken links & add link to HTML5 Living Standard.