If you are using html5 then you should use normal doctype.
<!DOCTYPE HTML>
<html>
<head>
<TITLE>My first HTML document</TITLE>
</head>
<body>
<input name="keyword" id="keyword" type="text" style="width:500px;" placeholder=" What" autocomplete="off" />
</body>
</html>
if you are using html4 or anything else then you can use following one of this
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0//EN">
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0//EN" "http://www.w3.org/TR/REC-html40/strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
For more info..