Question

I know that different doctypes are essentially about how compliant the html is, but what difference does it make what doctype you specify? Do browsers handle the same code differently depending on the doctype?
Thanks

UPDATE – most answers mention quirks mode can be set off if no doctype is specified. But what would be the different between xhtml and html 4.01?

Answer 1

The biggest thing is having a doctype or not. If you don’t, the browser will work in a “quirks” mode rather than standards mode and many things will be slightly different. If you have one — any — that typically activates more standards-compliant behavior in the browser.

See this article for the details of what doctypes do on various different browsers and what modes — quirks, standards, almost-standards, etc. — different browsers have. Quoting a relevant section:

Modes for text/html Content

The choice
of the mode for text/html content
depends on doctype sniffing (discussed
later in this document). In IE8 and
IE9, the mode also depends on other
factors. However, by default even in
IE8 and IE9, the mode depends on the
doctype for non-intranet sites that
are not on a blacklist supplied by
Microsoft.

It cannot be stressed
enough that the exact behavior of the
modes varies from browser to browser
even though discussion in this
document has been unified.

Quirks Mode

In the Quirks mode the
browsers violate contemporary Web
format specifications in order to
avoid “breaking” pages authored
according to practices that were
prevalent in the late 1990s. Different
browsers implement different quirks.
In Internet Explorer 6, 7, 8 and 9,
the Quirks mode is effectively frozen
IE 5.5. In other browsers, the Quirks
mode is a handful of deviations from
the Almost Standards mode.

If you are authoring new pages now,
you are supposed to comply with the
relevant specifications (CSS 2.1 in
particular) and use the Standards
mode.

Standards Mode

In the Standards mode
the browsers try to give conforming
documents the specification-wise
correct treatment to the extent
implemented in a particular browser.

Since different browsers are at
different stages of compliance, the
Standards mode isn’t a single target,
either.

HTML 5 calls this mode the “no quirks
mode”.

Almost Standards Mode

Firefox, Safari,
Chrome, Opera (since 7.5), IE8 and IE9
also have a mode known as “the Almost
Standards mode”, which implements the
vertical sizing of table cells
traditionally and not rigorously
according to the CSS2 specification.
Mac IE 5, Windows IE 6 and 7, Opera
prior to 7.5 and Konqueror do not need
an Almost Standards mode, because they
don’t implement the vertical sizing of
table cells rigorously according to
the CSS2 specification in their
respective Standards modes anyway. In
fact, their Standards modes are closer
to the Almost Standards mode than to
the Standards mode of newer browsers.

HTML 5 calls this mode the “limited
quirks mode”.

IE7 Mode

IE8 and IE9 have a mode that
is mostly a frozen copy of the mode
that was the Standards mode in IE7.
Other browsers do not have a mode like
this, and this mode is not specified
by HTML5.

IE8 Standards Mode

IE9 has a mode that
is mostly a frozen copy of the mode
that was the Standards mode in IE8.
Other browsers do not have a mode like
this, and this mode is not specified
by HTML5.

IE8 Almost Standards Mode

IE9 has a
mode that is mostly a frozen copy of
the mode that was the Almost Standards
mode in IE8. Other browsers do not
have a mode like this, and this mode
is not specified by HTML5.

…but see the article for a full discussion.