Every web browser has a built in set of CSS rules that are applied by default to every HTML document, called the user agent stylesheet. That is why when you look at a “raw” HTML document, it will still have some basic styling like large headers and blue, underlined links:
Unfortunately, browsers have slight differences in their user agent stylesheet, and due to the CSS Cascade, that can lead to inconsistencies in how your page renders in different browsers.
Without using a CSS reset, your page may look different in different browsers, which is a headache.
You just did. take a look at the source of one of the common resets to see exactly what they’re doing.