Specify a DOCTYPE for your HTML pages.
This tells the browser what version of HTML it is displaying.
There are several examples on the w3schools website.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> </head> </html>
For HTML5, use:
<!DOCTYPE html> <html> <head> </head> </html>