Web browsers have developed two modes, known as Standards mode and Quirks mode. These modes differentiate how web browsers handle new standards-compliant sites vs. older legacy sites. Browser layout engines now cooperate in one of three modes: quirks mode, almost standards mode, or full standards mode.  Quirks Mode: Many web browsers utilize a method called "Quirks mode" to display non-standards-compliant content so that it displays correctly on older browsers. Our page is likely running without a type declaration because the browser is attempting to interpret 'best guess' in Quirks mode, which is combined with a broad knowledge of code that may not be properly organized, non-standard, or written ineffectively. Quirks mode is basically just Internet Explorer 5 backward compatibility.  Almost Standards Mode: HTML and CSS standards require the use of the "standards mode," which is by default available in most modern web browsers. It renders HTML and CSS according to their respective DOCTYPEs. It performs syntax checks according to the DOCTYPE declared in the W3C Standard. A properly structured code is welcomed, and the desired actions are carried out.  Full Standard Mode: A further compatibility option, called full standard mode or strict mode, which implements a single non-standard behavior for table cell sizes while otherwise adhering to the requirements.