Let’s start with a quick review of the CSS box model.
Suppose I have a div. From the outside in, I first have the margin, which is distance from the surrounding elements. Then there’s a border. Inside the border is padding, and inside that is the content.
In a modern browser, the total width that the div takes up on the screen (from the border in) will be the combined widths of the content, padding, and border. For example, given a div with width of 80px, border 3px, padding 7px will take up 100 pixels (80 + 3 + 3 + 7 + 7), as specified by the CSS standard. In IE6, it will take up 80 pixels, as internet explorer considers the border and padding to be part of the specified width. Accordingly, if you use the same style sheet for all browsers, your divs are likely to look much thinner when viewed with IE6.
The other major problem related to the box model is known as the double margin bug; when a box is floated against the edge of the containing div, IE6 will double the margin on that edge. There are several hacks to fix this; without getting into why they work, just know that you can get rid of the error by adding one of two statements:
Neither should have any effect on other browsers; in fact, because zoom is a Microsoft-specific thing (nothing other than IE recognizes it), non-Microsoft browsers will ignore it completely (although it will make your style sheet fail a CSS standards check).