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>
.....
This usually happens because you are loading an HTML document as a script. This is often caused by <script src=""></script> (i.e. a relative URI pointing at the current, HTML, document)) or one of the scripts pointing to a 404 error.
DOCTYPE normally is expressed on one line like below. and can cause syntax error
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
But even sometimes it will yell about a syntax error when a src attribute is not declared for an element. Firebug can also through this error when a source is not found or is called twice on one page. EX: script/image/link.
来源：https://stackoverflow.com/questions/7714896/firebug-returns-syntax-error-in-doctype