The Doctype is an indication in an HTML page to define the type of syntax used in the page.
If the Doctype is not correctly defined, the browser may have difficulty displaying your page and display it in an unexpected way. This is called the Quirks mode of a browser.
Just add the "<!DOCTYPE html>" insctruction at the top of the HTML document :
<!DOCTYPE html> <html lang="en"> …
It is recommended to use the HTML 5 doctype, the other versions are considered "outdate".