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All major web browsers have a default stylesheet they use in the absence of an external stylesheet. These default stylesheets are known as user agent stylesheets. In this case, the term "user agent" is a technical term for the browser.

User agent stylesheets often have default CSS rules that set default values for padding and margin. This affects how the browser displays HTML elements, which can make it difficult for a developer to design or style a web page.

Many developers choose to reset these default values so that they can truly work with a clean slate.

* { margin: 0; padding: 0; }

The code in the example above resets the default margin and padding values of all HTML elements. It is often the first CSS rule in an external stylesheet.

Note that both properties are both set to 0. When these properties are set to 0, they do not require a unit of measurement.

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