Importance of Proper HTML Structure
HTML is very flexible so you would not get any error if missed any tag. However, sometimes you layout may break. Still most of CMS take care of this and autocomplete opened tags. It is very important to properly close HTML tags because of two reasons: It may misbehave in some browsers if tags are not properly closed. It may impact on your ranking (because of SEO). The reasons is that your page would not be considered W3C compliant. Therefore, whenever you write HTML from scratch, check your overall page structure and verify.