Browser: Layout engine

Rendering mode

Most web browsers's layout engines operate in one of three rendering modes:

Quirks mode
Limited-quirks mode (formerly: Almost standard mode)
No-quirks mode (formerly: Full standard mode)

Apparently, in more recent times, it makes no sense anymore to distinguish between Almost standard and standard mode.

The mode that a document is rendered in is controlled by the document's DOCTYPE declaration. The DOCTYPE declaration <!DOCTYPE html> specifies that a document follows no-quirks mode.

In JavaScript, the mode currently in effect can be determined with the property document.compatMode.

Root element

In no-quirks mode, the root element is document.documentElement (i. e. the HTML element).

In quirks mode, the root element is document.body (i. e. the BODY element) (update 2024-04-20: this assertion seems no longer true (if it ever was): in firefox, document.documentElement returned the HTML element even in quirks mode).