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>