A page looking right in one browser tells you how that browser handled your code. To recreate a compatibility check from the early web, take one small page and compare it in two period browsers, such as Internet Explorer 6 and Netscape 7. Use offline virtual machines if you have them available. Keep those old browsers away from the public web.

Use the same HTML and CSS files for both tests. Put a long heading above two columns, add an image with a caption, and give the navigation several links. At roughly the same window size, write down where the heading wraps, whether the columns fit, and whether any links are hidden or hard to read.

Change one rule, reload both browsers, and record what changed. If the heading still wraps differently but the page stays readable, you may be able to leave it alone. If the menu overlaps the article, reduce the example until you find the rule that causes it. The smaller page will be easier to test again.