Is IE the only browser that uses Quirks mode?

Question

When a DOCTYPE isn't defined or is mispelled, etc. are the older IEs the only browsers that go into quirks mode?

Or does super old Firefox, Safari, etc. do this as well?

Answer 1

No, browsers generally have a quirks mode. Each browser has its own quirks mode, though they share some of the behavior. There have been attempts to specify what exactly happens in quirks mode (more or less making it defined behavior), but the current situation is a mess: if you do not use a doctype string that triggers “standards mode”, you will get different quirks on different browsers.

See Activating Browser Modes with Doctype and What happens in Quirks Mode?

Answer 2

A short look into Wikipedia answers this...