The structure of a HTML document does not allow to insert all type of tags in all conditions.
For example, the structure of the HTML document contains some mandatory tags that are inserted in a defined order.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>Title</title> </head> <body> ... body of document ... </body> </html>
HTML specification: http://www.w3.org/TR/html4/