HTML and CSS Reference
There are several other considerations in rendering engines beyond standard compliance, for example, security or
CSS Selectors API  support.
Layout tests, Mochi tests, and RefTests can be performed by BrowserTests, a repository of browser test cases and
test suites .
A comprehensive comparison and evaluation of overall browser functionality are provided by BrowserScope.
It is a “community-driven project for profiling Web browsers” . The site provides up-to-date information on recent
tests performed on the latest browser versions. Browsers can be compared, and tests can be run on the browser used
for rendering the site.
Standards vs. Quirks Modes, DOCTYPE Switching
Standard-compliance problems of web browsers are not recent. The situation has been constantly improved,
however. After partially supporting the W3C Recommendations, browser users and web site developers faced a
serious problem. Millions of web sites developed earlier for older browsers looked fine in obsolete rendering engines
but had serious issues in the latest ones. In other words, compliance with W3C Recommendations became a problem.
In 1998, Todd Fahrner from the Web Standards Project invented the solution known as DOCTYPE switching .
Older, nonstandard documents with a missing DOCTYPE might produce different results in various rendering engines.
Modern browsers check the DOCTYPE , and if the expected behavior follows W3C standards, the document is rendered
in Standards Mode ( Strict Mode ). If the Document Type Definition is missing, browsers switch to a mode known as
Quirks Mode 8  that can deal with the nonstandard, unexpected behavior of older browsers (Figure 1-3 ).
Figure 1-3. W3C test file in Standards Mode  and Quirks Mode 
8 In Internet Explorer 9 and 10, the backward compatibility mode is called Compatibility Mode , which was renamed in Internet
Explorer 11 to Emulation , providing a mechanism that can also be triggered manually to render version-targeted web sites with
older versions of the rendering engine.