The first line of an HTML document should be the doctype tag. It tells the web browser which version of HTML or XHTML you are using.
HTML5
<!DOCTYPE html>
HTML 4.01
<<span class="token punctuation">!</span><span class="token doctype-tag">DOCTYPE</span> <span class="token name">HTML</span> <span class="token name">PUBLIC</span> <span class="token string">"-//W3C//DTD HTML 4.01 Transitional//EN"</span> <span class="token string">"http://www.w3.org/TR/html4/loose.dtd"></span>
XHTML 1.1
<<span class="token punctuation">!</span><span class="token doctype-tag">DOCTYPE</span> <span class="token name">html</span> <span class="token name">PUBLIC</span> <span class="token string">"-//W3C//DTD XHTML 1.1//EN"</span> <span class="token string">"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"</span>>