When in quirks mode the browser tries to be backward compatible by emulating rendering of the layout in older browsers. When in standard mode, the browser renders the page respecting all implemented standards. The modes mainly affect the CSS layout, but also HTML elements in certain older browsers.

To activate the standard mode just add the declaration below as the first line of your HTML document:

<!doctype html>

If you don’t include the declaration, the browser will run on quirks mode, but that’s not advisable as it could cause unexpected effects to your pages.