Why does Firebug return an error in my doctype?

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">

error

syntax error
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"

source code

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 
 "http://www.w3.org/TR/html4/strict.dtd">
<html>
    <head>
.....

html firebug

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