Valid, properly written HTML allows search engines to easily read your Website content and underlying HTML code. Much like the English language, there is a right way and a wrong way to write the code that structures your Web pages. The World Web Consortium (W3C), an international group that works to develop protocols and guidelines that ensure the long-term growth of the Web, had developed standards for HTML development. Web browser developers like Mozilla and Microsoft adhere to these standards to guarantee cross-browser uniformity when displaying websites.
Search engines read websites line by line and errors in your HTML syntax can trigger a penalty if the code prevents the search engine spider from reading the content. Invalid code can cause websites to look different from browser to browser.
Valid HTML also tends to reduce the overall amount of code on your website. Optimize you website my by not having excess markup surrounding your main page content. Less markup code means a higher percentage of your page size is actual content.
