What is a user agent stylesheet?
What are the target browsers? Different browsers set different default CSS rules. Try including a CSS reset, such as the meyerweb CSS reset or normalize.css, to remove those defaults. Google "CSS reset vs normalize" to see the differences.
If <!DOCTYPE> is missing in your HTML content you may experience that the browser gives preference to the "user agent stylesheet" over your custom stylesheet. Adding the doctype fixes this.
Regarding the concept “user agent style sheet”, consult section Cascade in the CSS 2.1 spec.
User agent style sheets are overridden by anything that you set in your own style sheet. They are just the rock bottom: in the absence of any style sheets provided by the page or by the user, the browser still has to render the content somehow, and the user agent style sheet just describes this.
So if you think you have a problem with a user agent style sheet, then you really have a problem with your markup, or your style sheet, or both (about which you wrote nothing).