If we don’t define a stylesheet and if our HTML doesn’t determine the look of an element, where does an element’s default CSS values come from?

Frequently Asked Questions CSS FAQ
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Question

This exercise says, “many properties in CSS have a default and don’t have to be set explicitly in the stylesheet.” If we don’t define a stylesheet and if our HTML doesn’t determine the look of an element, where does an element’s default CSS values come from?

Answer

An HTML element’s default CSS values come from a user agent style sheet. This style sheet is simply the set of rules that a browser applies to each web page. It’s important to realize that each browser has a slightly different user agent style sheet.