If you use features of the latest version of HTML, make sure that your Web pages work properly with older browsers. The user's browser and computer equipment. Note that you control only one of these factors. Browsers use HTML tags to determine how to present the document on the screen. Different browsers may display a given tag in different ways.
Browsers ignore all extra spaces, hard returns, tabs, indents, etc.
Use spaces, returns, tabs and indents to make your HTML code easier to read, but don't expect this spacing to carry over onto your Web pages. HTML has certain syntax rules that you should follow. Some browsers are more forgiving than others if the rules are not followed. Just because the document works with your browser doesn't mean it will work with someone else's. As HTML and browsers become more complex and sophisticated, it becomes more important to make sure you write valid HTML pages.