There are many other, perhaps more important, aspects of SEO. However, there is no time better than the present to make sure your website is free from missing pages and that it has clean html and css.
Never forget, google’s fundamental aim is to provide search results pointing to quality content that is on target with the posted search query.
So let’s look at a few things that will help with the “quality” part.
Fix Missing Pages & Broken Links
404s (missing pages) are a bad thing. They are disappointing, and generally are to be avoided. One way that google has of helping people avoid 404s is to lower the page ranking of web pages found on sites with an abundance of 404s. Why should google put your site on the first page of search results when there is a good chance that the page is missing?
Spell Check Your Pages
Why should google be sticking their nose into the business of spelling and other such content issues? Hey, remember that part about providing search results leading to quality content. Well, google is trying many things in order to identify quality content. I don’t believe they have spelling as part of their algorithm at the moment. But, what if one of the misspelled words is a keyword? No good for you. Also the kind of attention/effort that goes into spell check and cleaning the written content is helpful for noticing other aspects of the page that would benefit from correction.
Sometimes the best approach is from the side. Reading your content for spelling and grammar errors also gives you an opportunity to notice old prices, bland or uninspiring ad copy — any number of items that would benefit from correction.
Also, people are more likely to create links to your pages if they are not put off by obvious typos and sloppy content. And remember, links to your site count a great deal toward the page ranking.
Validation
I use the W3.org HTML Validator and the W3.org CSS Validator. There are others. Some may be far superior. I just have developed a habit of using these.
Why validate? Because, broken html and broken css will result in broken pages. There is a thing called cross-browser compatibility. Just because Firefox or Chrome somehow manages to render the tangled mess of html and css that you call a page, does not mean that IE, or Safari, or Opera, or any number of other browsers will. You want your page rendered reasonably well on many different platforms.
Also, the attention needed to fix html and fix css puts you in the right frame of mind for general refinement of your website.
Does food coming from a clean kitchen taste better? Maybe yes, maybe no. But which would you prefer to eat?