The browser has a basic style sheet that gives a default style to any document. These style sheets are named user-agent stylesheets. Some browsers use actual style sheets for this purpose, while others simulate them in code, but the end result is the same.
Some browsers let users modify the user-agent stylesheet. Although some constraints on user-agent stylesheets are set by the HTML specification, browsers still have a lot of latitude: that means that significant differences exist from one browser to another. To simplify the development process, Web developers often use a CSS reset style sheet, forcing common properties values to a known state before beginning to make alterations to suit their specific needs.
Author stylesheets are style sheets that define styles as part of the design of a given web page or site. The author of the page defines the styles for the document using one or more stylesheets, which define the look and feel of the website — its theme
The user (or reader) of the website can choose to override styles in many browsers using a custom user stylesheet designed to tailor the experience to the user's wishes.
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