CSS parsing, in which a number of illegal CSS statements are present in Acid2 to test error handling. Standards-compliant browsers are expected to handle these errors as the CSS specification directs. This helps ensure cross-browser compatibility by making all browsers treat CSS with the same level of strictness, so that what works in one browser should not cause errors in another. Paint order, which means that Acid2 requires that the browser has standard paint order. That is, overlapping elements should be placed or "painted" on top of each other in the correct order, and finally Hovering effects, which is When the user moves his or her mouse over the smiley face's nose, it turns blue. This is called a hovering effect, and while it has traditionally been used for hyperlinks, it should work on a wide variety of HTML elements.