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:

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.