Hyper Text Markup Language
HTML is the predominant markup language for web pages. HTML elements are the basic elements of webpages including the document text and any hyperlinks and images embedded within.
HTML is written in the form of HTML elements consisting of tags, enclosed in angle brackets. HTML tags normally come in pairs like <h1> and </h1>. The first tag in a pair is the start tag, the second tag is the end tag (they are also called opening tags and closing tags). In between these tags you can can add text, tables, images, etc..
HTML errors may cause the page to appear broken to (some) browser users. It may also cause mistakes in the analysis of the page by Ranks NL and search engines such as Google.
Google Do
Write good, clean HTML. While your site may appear correctly in some browsers even if your HTML is not valid, there's no guarantee that it will appear correctly in all browsers - or in all future browsers. The best way to make sure that your page looks the same in all browsers is to write your page using valid HTML and CSS, and then test it in as many browsers as possible.