There are a lot of little things that can junk up your code, but, for the most part, the search engines don’t really care. It doesn’t matter to them if your HTML validates or if you keep your code “clean” or not. When it comes to analyzing your pages for search engine rankings, none of that really matters a whole lot. But when it does matter, it matters a whole lot.
Here are some typical coding issues that can screw up your on-page optimization processes: