Often, one of the most painful exercises in web development is simply getting everything to look consistent across the major browser platforms. No standard states how a particular HTML element should look. The intent of a markup might be self-evident; we would expect an <h1> to look larger or bolder than an <h2>, and both larger than plain text but there is no standard saying by how much. Developers will often design and test an implementation in one particular browser, and then when everything is working and rendering correctly in their browser of choice, they will have to refine it and tweak it until it works and renders similarly in all of the other major browsers.
Due to the differences between the default stylesheet in use in the various browsers, getting pages to render in the same way can be a challenge. Something that lines up perfectly in IE will undoubtedly be misaligned in other browsers. And of course, when you correct the differences...