Inclusion of a DocType declaration is simple. A statement such as one of the following is included on the first line of your html document, even before the <html> tag.
DocType Declarations for HTML 4.01 Strict, Transitional, and Frameset
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
See the references if you are not writing Html 4.01.
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References on Document Types:
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To be completely correct, you should tell the browser what character set you are using. This is very helpful if you write in languages with non arabic characters. A tag such as the following can do the job:
<META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
It is included as early in the file as possible, typically right after the <html> tag.
Go to this site to have W3C validate your page: W3 Validation Service