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<!DOCTYPE> declaration. The declaration is not an HTML tag. It is an "information" to the browser about what document type to expect. In HTML5, the <!DOCTYPE> declaration is simple: <!DOCTYPE html> In older documents (HTML 4 or XHTML), the declaration is more complicated because the declaration must refer to a DTD (Document Type Definition).
Formula
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">You can read more about document types in the <!DOCTYPE> reference.
<a>
<abbr>
<acronym> No
<address>
<applet> No
No <area>
No <article>
No No <aside>
No No <audio>
No No <b>