In chapter 8 it talks about how there are many common errors in modern markup. Basically that means that there are people making things much harder on themselves than it has to be, and eventually it might effect the design. For example in the book it shows a class in front of each paragraph but really all you have to have are those paragraphs wrapped in a div. You do not want to overuse the div because that can also create a mess just like it would if you used the classes. One of the best ways to keep things labeled is to use an id. You can use id’s for many things such as ul id=”navigation” or div id=”books”(examples from the book). Another habit that you shouldn’t get into is inline CSS. CSS should be done in a stylesheet and not in the markup.
You should always test your design in a browser such as Firefox, Opera, or Safari. If your design looks like it should in one of those web browsers then you should test it on the latest version of Internet Explorer. If they do not look the same you need to go back into your CSS. By checking your design in each of these browsers you eliminate the errors.
These are just some helpful tips that will help in making a better website, and doing things in a way that will keep up with modern times. A good way to start is not getting into bad habits that can create more problems in the long run for you.
http://blog.themeforest.net/tutorials/common-mistakes-in-web-design/ (copy and Paste)