Explain almost standard, full standard and quirks mode

There are now three modes used by the layout engines in web browsers: quirks mode, almost standards mode, and full standards mode.

• In quirks mode, layout emulates nonstandard behavior in Navigator 4 and Internet Explorer 5. This is essential in order to support websites that were built before the widespread adoption of web standards.
• In full standards mode, the behavior is (hopefully) the behavior described by the HTML and CSS specifications.
• In almost standards mode, there are only a very small number of quirks implemented.

For HTML documents, browsers use a DOCTYPE in the beginning of the document to decide whether to handle it in quirks mode or standards mode.