New Doctype Sniffing In Upcoming Mozilla Releases

The Mozilla browser, like several other browsers on the market, uses DOCTYPE sniffing to determine how to render a page. In Mozilla's case, it either renders the page strictly, in "standards mode," or more loosely -- otherwise known as "quirks mode." Now Mozilla introduces something more.

Almost Standards

In the upcoming 1.01 and 1.1 releases, Mozilla will add an "almost standards" mode to its mix. This mode is virtually identical to the standards mode (now being referred to as "full standards mode") but with one crucial change. In almost standards mode, Mozilla will not implement the CSS-2 line-height rules that causes many pages with pixel-precise image layouts via tables to break apart.

DOCTYPE Changes

Several DOCTYPEs that formerly triggered full standards mode will now render in almost standards mode. Those DOCTYPEs are:

• XHTML 1.0 Transitional
• XHTML 1.0 Frameset
• HTML 4.01 Transitional with a system identifier URL in the DOCTYPE
• HTML 4.01 Frameset with a system identifier URL in the DOCTYPE
• One particular system DOCTYPE as a concession to some specific needs of IBM (all other system DOCTYPEs render in full standards mode).

Reference material

• Bugzilla discussion of how/why the CSS-2 line-height/images in tables problem needed to be addressed in some fashion.
• Bugzilla bug that documents the creation of almost standards mode.
• Mozilla's list of what DOCTYPEs trigger what mode.
• Eric Meyer's documentation of the line-height/images in tables issue.