A user-agent stylesheet (browser default styles) is a set of default CSS rules that web browsers apply to HTML elements automatically, ensuring consistent visual rendering across different web pages. These pre-defined styles exist in every browser and help maintain uniformity for elements like headings, paragraphs, lists, and more, by applying basic styles like margins, padding, font sizes, and colors.
(You may remove the differnece between browsers via reset stylesheet such as normalize.css)
The user may be able to specify style information for a particular document. For example, the user may specify a file that contains a style sheet or the user agent may provide an interface that generates a user style sheet (or behaves as if it did).
(for instance, the safari user may add a stylesheet to the browser via: settings > advanced > stylesheets)
Author stylesheets are the most common type of stylesheet; these are the styles written by web developers. These styles can reset user-agent styles, as noted above, and define the styles for the design of a given web page or application. The author, or web developer, defines the styles for the document using one or more linked or imported stylesheets, <style> blocks, and inline styles defined with the style attribute. These author styles define the look and feel of the website — its theme.