<s>
A	O
well-formed	B-Language
document	I-Language
in	O
XML	B-Protocol
is	O
a	O
document	O
that	O
"	O
adheres	O
to	O
the	O
syntax	B-Application
rules	O
specified	O
by	O
the	O
XML	B-Protocol
1.0	O
specification	O
in	O
that	O
it	O
must	O
satisfy	O
both	O
physical	O
and	O
logical	O
structures	O
"	O
.	O
</s>
<s>
At	O
its	O
base	O
level	O
well-formed	B-Language
documents	I-Language
require	O
that	O
:	O
</s>
<s>
To	O
be	O
a	O
well-formed	B-Language
document	I-Language
,	O
rules	O
must	O
be	O
established	O
about	O
the	O
declaration	O
and	O
treatment	O
of	O
entities	O
.	O
</s>
<s>
Tags	B-General_Concept
are	O
case	O
sensitive	O
,	O
with	O
attributes	O
delimited	O
with	O
quotation	O
marks	O
.	O
</s>
<s>
Overlapping	O
tags	B-General_Concept
invalidate	O
a	O
document	O
.	O
</s>
<s>
Ideally	O
,	O
a	O
well-formed	B-Language
document	I-Language
conforms	O
to	O
the	O
design	O
goals	O
of	O
XML	B-Protocol
.	O
</s>
<s>
Other	O
key	O
syntax	B-Application
rules	O
provided	O
in	O
the	O
specification	O
include	O
:	O
</s>
<s>
None	O
of	O
the	O
special	O
syntax	B-Application
characters	O
such	O
as	O
<	O
and	O
&	O
appear	O
except	O
when	O
performing	O
their	O
markup-delineation	O
roles	O
.	O
</s>
<s>
The	O
begin	O
,	O
end	O
,	O
and	O
empty-element	O
tags	B-General_Concept
that	O
delimit	O
the	O
elements	O
are	O
correctly	O
nested	O
,	O
with	O
none	O
missing	O
and	O
none	O
overlapping	O
.	O
</s>
<s>
The	O
element	O
tags	B-General_Concept
are	O
case-sensitive	O
;	O
the	O
beginning	O
and	O
end	O
tags	B-General_Concept
must	O
match	O
exactly	O
.	O
</s>
<s>
Tag	B-General_Concept
names	O
cannot	O
contain	O
any	O
of	O
the	O
characters	O
!	O
</s>
<s>
A	O
valid	B-Protocol
XML	I-Protocol
document	I-Protocol
is	O
defined	O
in	O
the	O
XML	B-Protocol
specification	I-Protocol
as	O
a	O
well-formed	B-Language
XML	I-Language
document	I-Language
which	O
also	O
conforms	O
to	O
the	O
rules	O
of	O
a	O
Document	B-Language
Type	I-Language
Definition	I-Language
(	O
DTD	O
)	O
.	O
</s>
<s>
According	O
to	O
JavaCommerce.com	O
XML	B-Protocol
tutorial	O
,	O
"	O
Well	O
formed	O
XML	B-Protocol
documents	I-Protocol
simply	O
markup	O
pages	O
with	O
descriptive	O
tags	B-General_Concept
.	O
</s>
<s>
You	O
do	O
n't	O
need	O
to	O
describe	O
or	O
explain	O
what	O
these	O
tags	B-General_Concept
mean	O
.	O
</s>
<s>
In	O
other	O
words	O
a	O
well	O
formed	O
XML	B-Protocol
document	I-Protocol
does	O
not	O
need	O
a	O
DTD	O
,	O
but	O
it	O
must	O
conform	O
to	O
the	O
XML	B-Protocol
syntax	B-Application
rules	O
.	O
</s>
<s>
If	O
all	O
tags	B-General_Concept
in	O
a	O
document	O
are	O
correctly	O
formed	O
and	O
follow	O
XML	B-Protocol
guidelines	O
,	O
then	O
a	O
document	O
is	O
considered	O
as	O
well	O
formed.	O
"	O
</s>
<s>
An	O
XML	B-Protocol
processor	O
that	O
encounters	O
a	O
violation	O
of	O
the	O
well-formedness	O
rules	O
is	O
required	O
to	O
report	O
such	O
errors	O
and	O
to	O
cease	O
normal	O
processing	O
.	O
</s>
<s>
This	O
policy	O
,	O
occasionally	O
referred	O
to	O
as	O
draconian	O
,	O
stands	O
in	O
notable	O
contrast	O
to	O
the	O
behavior	O
of	O
programs	O
that	O
process	O
HTML	B-Language
,	O
which	O
are	O
designed	O
to	O
produce	O
a	O
reasonable	O
result	O
even	O
in	O
the	O
presence	O
of	O
severe	O
markup	O
errors	O
in	O
the	O
spirit	O
of	O
Postel	O
's	O
law	O
(	O
"	O
Be	O
conservative	O
in	O
what	O
you	O
send	O
;	O
be	O
liberal	O
in	O
what	O
you	O
accept	O
"	O
)	O
.	O
</s>
<s>
The	O
concept	O
of	O
a	O
well-formed	B-Language
document	I-Language
allows	O
for	O
a	O
better	O
understanding	O
of	O
the	O
fundamental	O
construction	O
of	O
XML	B-Protocol
.	O
</s>
<s>
It	O
helps	O
to	O
clarify	O
XML	B-Protocol
beyond	O
the	O
typical	O
sense	O
of	O
it	O
.	O
</s>
<s>
For	O
example	O
,	O
while	O
most	O
XML	B-Protocol
Document	I-Protocol
Type	O
Definitions	O
utilize	O
left	O
and	O
right	O
angle	O
brackets	O
as	O
content	O
delimiters	O
,	O
strictly	O
speaking	O
this	O
is	O
not	O
a	O
necessity	O
(	O
though	O
a	O
delimiter	O
should	O
be	O
terse	O
and	O
concise	O
)	O
.	O
</s>
<s>
The	O
concept	O
of	O
well-formed	B-Language
document	I-Language
also	O
allows	O
for	O
the	O
comprehension	O
of	O
the	O
abstract	O
nature	O
of	O
XML	B-Protocol
.	O
</s>
<s>
In	O
reality	O
,	O
there	O
is	O
no	O
such	O
thing	O
as	O
XML	B-Protocol
.	O
</s>
<s>
Rather	O
,	O
XML	B-Protocol
is	O
a	O
principle	O
that	O
represents	O
a	O
set	O
of	O
behaviors	O
and	O
practices	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
discuss	O
types	O
of	O
XML	B-Protocol
,	O
as	O
expressed	O
within	O
a	O
Document	B-Language
Type	I-Language
Definition	I-Language
(	O
DTD	O
)	O
.	O
</s>
<s>
Well-formed	B-Language
documents	I-Language
also	O
bring	O
into	O
focus	O
the	O
issue	O
of	O
valid	O
versus	O
correct	O
XML	B-Protocol
.	O
</s>
<s>
Thus	O
,	O
tags	B-General_Concept
or	O
entities	O
must	O
be	O
in	O
conformity	O
to	O
the	O
rules	O
and	O
relations	O
established	O
within	O
a	O
DTD	O
.	O
</s>
<s>
However	O
,	O
there	O
is	O
no	O
control	O
on	O
whether	O
a	O
tag	B-General_Concept
or	O
entity	O
is	O
correct	O
.	O
</s>
<s>
Thus	O
a	O
first	O
level	O
head	O
tag	B-General_Concept
could	O
be	O
applied	O
to	O
a	O
second	O
level	O
head	O
object	O
and	O
be	O
valid	O
,	O
while	O
incorrect	O
.	O
</s>
<s>
The	O
emphasis	O
on	O
well-formed	B-Language
documents	I-Language
has	O
developed	O
within	O
the	O
publishing	O
industry	O
where	O
the	O
use	O
of	O
left	O
and	O
right	O
angle	O
bracket	O
delimited	O
information	O
has	O
become	O
problematic	O
.	O
</s>
<s>
Emphasis	O
on	O
the	O
well-formed	B-Language
document	I-Language
allows	O
for	O
the	O
definition	O
,	O
delimiting	O
,	O
and	O
nesting	B-General_Concept
of	O
content	O
to	O
be	O
managed	O
within	O
programs	O
that	O
are	O
not	O
XML	B-Protocol
,	O
per	O
se	O
,	O
but	O
exhibit	O
the	O
characteristics	O
or	O
potential	O
for	O
being	O
well	O
formed	O
.	O
</s>
<s>
There	O
are	O
several	O
tools	O
available	O
to	O
determine	O
if	O
a	O
given	O
XML	B-Protocol
document	I-Protocol
is	O
well	O
formed	O
.	O
</s>
