Weird behaviour from !DOCTYPE [closed]

1 day ago 2
ARTICLE AD BOX

I'm making the homepage of a website, but for some reason whenever I put the !DOCTYPE a weird barrier pushes everything down.

I don't understand what's causing it and how to fix it, can someone help?

P.S. I've already set body and html' height and width to 100%

<!DOCTYPE html> <html> <head> <link rel="stylesheet" href="styles.css"> <header style="margin-left:-10px; margin-right: -10px; margin-top: -10px;"> <h1 style="color: #c9a7c7;background-color: #4e3e50; text-align: center; font-size: 200px; font-family:'Times New Roman', Times, serif; margin-bottom: 0;" >Ticket</h1> <h3 style="color: #663366;background-color: #c9a7c7; margin-top: 0; margin-left: 20px;"><strong style="font-weight: normal;"> &nbsp; <a href="login.html">User</a> &nbsp; <a href="eventi.html">Eventi</a> &nbsp; <a href="location.html">Location</a> <strong> &nbsp; Cerca <input type="search"> </strong> </strong></h3> </header> </head> <body style="background-color: #c9a7c7; color: #4e3e50; margin-bottom: 20px;"> </body> </html>
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