The markup i.e., (X)HTML of your web page exists in many versions such as: HTML 4.01 Strict, XHTML 1.0 Transitional, XHTML 1.0 Strict etc. A web browser parses your web page and renders the content based on the version in which your markup exists. This information will be available to the browser though the DOCTYPE declaration. It should be declared before the <html> tag and looks like :
<!DOCTYPE html PUBLIC "-//W3C//DTD
XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Omitting DOCTYPE in the markup is one of the main reasons for cross browser compatibility issues. Sometimes, even if your mark up is valid (with XHTML and CSS), there might still be problems due to incorrect DOCTYPE.
Please go through these excellent links which explain the importance and implementation of DOCTYPE.