<s>
A	O
document	B-Language
type	I-Language
definition	I-Language
(	O
DTD	O
)	O
is	O
a	O
set	O
of	O
markup	O
declarations	O
that	O
define	O
a	O
document	O
type	O
for	O
an	O
SGML-family	O
markup	O
language	O
(	O
GML	B-Application
,	O
SGML	B-Language
,	O
XML	B-Protocol
,	O
HTML	B-Language
)	O
.	O
</s>
<s>
A	O
DTD	O
defines	O
the	O
valid	O
building	O
blocks	O
of	O
an	O
XML	B-Protocol
document	I-Protocol
.	O
</s>
<s>
A	O
DTD	O
can	O
be	O
declared	O
inline	O
inside	O
an	O
XML	B-Protocol
document	I-Protocol
,	O
or	O
as	O
an	O
external	O
reference	O
.	O
</s>
<s>
XML	B-Protocol
uses	O
a	O
subset	O
of	O
SGML	B-Language
DTD	O
.	O
</s>
<s>
,	O
newer	O
XML	O
namespace-aware	O
schema	O
languages	O
(	O
such	O
as	O
W3C	O
XML	B-Language
Schema	I-Language
and	O
ISO	B-Language
RELAX	I-Language
NG	I-Language
)	O
have	O
largely	O
superseded	O
DTDs	O
.	O
</s>
<s>
A	O
namespace-aware	O
version	O
of	O
DTDs	O
is	O
being	O
developed	O
as	O
Part	O
9	O
of	O
ISO	O
DSDL	B-Language
.	O
</s>
<s>
DTDs	O
persist	O
in	O
applications	O
that	O
need	O
special	O
publishing	O
characters	O
,	O
such	O
as	O
the	O
XML	B-General_Concept
and	I-General_Concept
HTML	I-General_Concept
Character	I-General_Concept
Entity	I-General_Concept
References	I-General_Concept
,	O
which	O
derive	O
from	O
larger	O
sets	O
defined	O
as	O
part	O
of	O
the	O
ISO	B-Language
SGML	I-Language
standard	I-Language
effort	O
.	O
</s>
<s>
A	O
DTD	O
is	O
associated	O
with	O
an	O
XML	B-Protocol
or	O
SGML	B-Language
document	O
by	O
means	O
of	O
a	O
document	O
type	O
declaration	O
(	O
DOCTYPE	O
)	O
.	O
</s>
<s>
The	O
DOCTYPE	O
appears	O
in	O
the	O
syntactic	O
fragment	O
doctypedecl	O
near	O
the	O
start	O
of	O
an	O
XML	B-Protocol
document	I-Protocol
.	O
</s>
<s>
The	O
external	O
subset	O
may	O
be	O
referenced	O
via	O
a	O
public	O
identifier	O
and/or	O
a	O
system	B-Language
identifier	I-Language
.	O
</s>
<s>
Any	O
valid	O
SGML	B-Language
or	O
XML	B-Protocol
document	I-Protocol
that	O
references	O
an	O
external	O
subset	O
in	O
its	O
DTD	O
,	O
or	O
whose	O
body	O
contains	O
references	O
to	O
parsed	B-Language
external	O
entities	O
declared	O
in	O
its	O
DTD	O
(	O
including	O
those	O
declared	O
within	O
its	O
internal	O
subset	O
)	O
,	O
may	O
only	O
be	O
partially	O
parsed	B-Language
but	O
cannot	O
be	O
fully	O
validated	O
by	O
validating	O
SGML	B-Language
or	O
XML	B-Protocol
parsers	I-Protocol
in	O
their	O
standalone	O
mode	O
(	O
this	O
means	O
that	O
these	O
validating	O
parsers	B-Language
do	O
n't	O
attempt	O
to	O
retrieve	O
these	O
external	O
entities	O
,	O
and	O
their	O
replacement	O
text	O
is	O
not	O
accessible	O
)	O
.	O
</s>
<s>
However	O
,	O
such	O
documents	O
are	O
still	O
fully	O
parsable	B-Language
in	O
the	O
non-standalone	O
mode	O
of	O
validating	O
parsers	B-Language
,	O
which	O
signals	O
an	O
error	O
if	O
it	O
ca	O
n't	O
locate	O
these	O
external	O
entities	O
with	O
their	O
specified	O
public	O
identifier	O
(	O
FPI	O
)	O
or	O
system	B-Language
identifier	I-Language
(	O
a	O
URI	B-Protocol
)	O
,	O
or	O
are	O
inaccessible	O
.	O
</s>
<s>
(	O
Notations	O
declared	O
in	O
the	O
DTD	O
are	O
also	O
referencing	O
external	O
entities	O
,	O
but	O
these	O
unparsed	O
entities	O
are	O
not	O
needed	O
for	O
the	O
validation	O
of	O
documents	O
in	O
the	O
standalone	O
mode	O
of	O
these	O
parsers	B-Language
:	O
the	O
validation	O
of	O
all	O
external	O
entities	O
referenced	O
by	O
notations	O
is	O
left	O
to	O
the	O
application	O
using	O
the	O
SGML	B-Language
or	O
XML	B-Protocol
parser	I-Protocol
)	O
.	O
</s>
<s>
Non-validating	O
parsers	B-Language
may	O
eventually	O
attempt	O
to	O
locate	O
these	O
external	O
entities	O
in	O
the	O
non-standalone	O
mode	O
(	O
by	O
partially	O
interpreting	O
the	O
DTD	O
only	O
to	O
resolve	O
their	O
declared	O
parsable	B-Language
entities	O
)	O
,	O
but	O
do	O
not	O
validate	O
the	O
content	O
model	O
of	O
these	O
documents	O
.	O
</s>
<s>
The	O
following	O
example	O
of	O
a	O
DOCTYPE	O
contains	O
both	O
public	O
and	O
system	B-Language
identifiers	I-Language
:	O
</s>
<s>
All	O
HTML	B-Language
4.01	I-Language
documents	O
conform	O
to	O
one	O
of	O
three	O
SGML	B-Language
DTDs	O
.	O
</s>
<s>
The	O
system	B-Language
identifiers	I-Language
of	O
these	O
DTDs	O
,	O
if	O
present	O
in	O
the	O
DOCTYPE	O
,	O
are	O
URI	B-Protocol
references	I-Protocol
.	O
</s>
<s>
A	O
system	B-Language
identifier	I-Language
usually	O
points	O
to	O
a	O
specific	O
set	O
of	O
declarations	O
in	O
a	O
resolvable	O
location	O
.	O
</s>
<s>
SGML	B-Language
allows	O
mapping	O
public	O
identifiers	O
to	O
system	B-Language
identifiers	I-Language
in	O
catalogs	B-Language
that	O
are	O
optionally	O
available	O
to	O
the	O
URI	B-Protocol
resolvers	O
used	O
by	O
document	O
parsing	B-Language
software	O
.	O
</s>
<s>
This	O
DOCTYPE	O
can	O
only	O
appear	O
after	O
the	O
optional	O
XML	B-Protocol
declaration	O
,	O
and	O
before	O
the	O
document	O
body	O
,	O
if	O
the	O
document	O
syntax	O
conforms	O
to	O
XML	B-Protocol
.	O
</s>
<s>
This	O
includes	O
XHTML	B-Language
documents	O
:	O
</s>
<s>
Finally	O
,	O
the	O
document	B-Language
type	I-Language
definition	I-Language
may	O
include	O
no	O
subset	O
at	O
all	O
;	O
in	O
that	O
case	O
,	O
it	O
just	O
specifies	O
that	O
the	O
document	O
has	O
a	O
single	O
top-level	O
element	O
(	O
this	O
is	O
an	O
implicit	O
requirement	O
for	O
all	O
valid	O
XML	B-Protocol
and	O
HTML	B-Language
documents	O
,	O
but	O
not	O
for	O
document	O
fragments	O
or	O
for	O
all	O
SGML	B-Language
documents	O
,	O
whose	O
top-level	O
elements	O
may	O
be	O
different	O
from	O
the	O
implied	O
root	O
element	O
)	O
,	O
and	O
it	O
indicates	O
the	O
type	O
name	O
of	O
the	O
root	O
element	O
:	O
</s>
<s>
DTD	O
markup	O
declarations	O
declare	O
which	O
element	O
types	O
,	O
attribute	O
lists	O
,	O
entities	O
,	O
and	O
notations	O
are	O
allowed	O
in	O
the	O
structure	O
of	O
the	O
corresponding	O
class	O
of	O
XML	B-Protocol
documents	I-Protocol
.	O
</s>
<s>
A	O
valid	B-Protocol
XML	I-Protocol
document	I-Protocol
contains	O
only	O
elements	O
that	O
are	O
defined	O
in	O
the	O
DTD	O
.	O
</s>
<s>
:	O
historically	O
meaning	O
parsed	B-Language
character	O
data	O
,	O
this	O
means	O
that	O
only	O
one	O
text	O
element	O
is	O
allowed	O
in	O
the	O
content	O
(	O
no	O
quantifier	O
is	O
allowed	O
)	O
;	O
</s>
<s>
Element	O
type	O
declarations	O
are	O
ignored	O
by	O
non-validating	O
SGML	B-Language
and	O
XML	B-Protocol
parsers	I-Protocol
(	O
in	O
which	O
cases	O
,	O
any	O
elements	O
are	O
accepted	O
in	O
any	O
order	O
,	O
and	O
in	O
any	O
number	O
of	O
occurrences	O
in	O
the	O
parsed	B-Language
document	O
)	O
,	O
but	O
these	O
declarations	O
are	O
still	O
checked	O
for	O
form	O
and	O
validity	O
.	O
</s>
<s>
Here	O
are	O
some	O
attribute	O
types	O
supported	O
by	O
both	O
SGML	B-Language
and	O
XML	B-Protocol
:	O
</s>
<s>
the	O
effective	O
value	O
of	O
the	O
attribute	O
must	O
be	O
a	O
valid	O
identifier	O
,	O
and	O
it	O
is	O
used	O
to	O
define	O
and	O
anchor	O
to	O
the	O
current	O
element	O
the	O
target	O
of	O
references	O
using	O
this	O
defined	O
identifier	O
(	O
including	O
as	O
document	O
fragment	O
identifiers	O
that	O
may	O
be	O
specified	O
at	O
end	O
of	O
an	O
URI	B-Protocol
after	O
a	O
"	O
#	O
"	O
sign	O
)	O
;	O
it	O
is	O
an	O
error	O
if	O
distinct	O
elements	O
in	O
the	O
same	O
document	O
are	O
defining	O
the	O
same	O
identifier	O
;	O
the	O
uniqueness	O
constraint	O
also	O
implies	O
that	O
the	O
identifier	O
itself	O
carries	O
no	O
other	O
semantics	O
and	O
that	O
identifiers	O
must	O
be	O
treated	O
as	O
opaque	O
in	O
applications	O
;	O
XML	B-Protocol
also	O
predefines	O
the	O
standard	O
pseudo-attribute	O
""	O
with	O
this	O
type	O
,	O
without	O
needing	O
any	O
declaration	O
in	O
the	O
DTD	O
,	O
so	O
the	O
uniqueness	O
constraint	O
also	O
applies	O
to	O
these	O
defined	O
identifiers	O
when	O
they	O
are	O
specified	O
anywhere	O
in	O
a	O
XML	B-Protocol
document	I-Protocol
.	O
</s>
<s>
or	O
the	O
effective	O
value	O
of	O
the	O
attribute	O
can	O
only	O
be	O
a	O
valid	O
identifier	O
(	O
or	O
a	O
space-separated	O
list	O
of	O
such	O
identifiers	O
)	O
and	O
must	O
be	O
referencing	O
the	O
unique	O
element	O
defined	O
in	O
the	O
document	O
with	O
an	O
attribute	O
declared	O
with	O
the	O
type	O
in	O
the	O
DTD	O
(	O
or	O
the	O
unique	O
element	O
defined	O
in	O
an	O
XML	B-Protocol
document	I-Protocol
with	O
a	O
pseudo-attribute	O
""	O
)	O
and	O
whose	O
effective	O
value	O
is	O
the	O
same	O
identifier	O
;	O
</s>
<s>
or	O
the	O
effective	O
value	O
of	O
the	O
attribute	O
can	O
only	O
be	O
the	O
name	O
of	O
an	O
unparsed	O
external	O
entity	O
(	O
or	O
a	O
space-separated	O
list	O
of	O
such	O
names	O
)	O
,	O
which	O
must	O
also	O
be	O
declared	O
in	O
the	O
document	O
type	O
declaration	O
;	O
this	O
type	O
is	O
not	O
supported	O
in	O
HTML	B-Language
parsers	B-Language
,	O
but	O
is	O
valid	O
in	O
SGML	B-Language
and	O
XML	B-Protocol
1.0	O
or	O
1.1	O
(	O
including	O
XHTML	B-Language
and	O
SVG	O
)	O
;	O
</s>
<s>
the	O
effective	O
value	O
of	O
the	O
attribute	O
can	O
only	O
be	O
any	O
one	O
of	O
the	O
enumerated	O
list	O
(	O
specified	O
between	O
parentheses	O
and	O
separated	O
by	O
a	O
""	O
pipe	O
character	O
)	O
of	O
notation	O
names	O
,	O
where	O
each	O
notation	O
name	O
in	O
the	O
enumeration	O
must	O
also	O
be	O
declared	O
in	O
the	O
document	O
type	O
declaration	O
;	O
this	O
type	O
is	O
not	O
supported	O
in	O
HTML	B-Language
parsers	B-Language
,	O
but	O
is	O
valid	O
in	O
SGML	B-Language
and	O
XML	B-Protocol
1.0	O
or	O
1.1	O
(	O
including	O
XHTML	B-Language
and	O
SVG	O
)	O
.	O
</s>
<s>
A	O
default	O
value	O
can	O
define	O
whether	O
an	O
attribute	O
must	O
occur	O
(	O
)	O
or	O
not	O
(	O
)	O
,	O
or	O
whether	O
it	O
has	O
a	O
fixed	O
value	O
(	O
)	O
,	O
or	O
which	O
value	O
should	O
be	O
used	O
as	O
a	O
default	O
value	O
("…"	O
)	O
in	O
case	O
the	O
given	O
attribute	O
is	O
left	O
out	O
in	O
an	O
XML	B-Protocol
tag	O
.	O
</s>
<s>
Attribute	O
list	O
declarations	O
are	O
ignored	O
by	O
non-validating	O
SGML	B-Language
and	O
XML	B-Protocol
parsers	I-Protocol
(	O
in	O
which	O
cases	O
any	O
attribute	O
is	O
accepted	O
within	O
all	O
elements	O
of	O
the	O
parsed	B-Language
document	O
)	O
,	O
but	O
these	O
declarations	O
are	O
still	O
checked	O
for	O
well-formedness	O
and	O
validity	O
.	O
</s>
<s>
Entities	O
help	O
to	O
improve	O
legibility	O
of	O
an	O
XML	B-Protocol
text	O
.	O
</s>
<s>
Internal	O
(	O
parsed	B-Language
)	O
entities	O
are	O
associating	O
a	O
name	O
with	O
any	O
arbitrary	O
textual	O
content	O
defined	O
in	O
their	O
declaration	O
(	O
which	O
may	O
be	O
in	O
the	O
internal	O
subset	O
or	O
in	O
the	O
external	O
subset	O
of	O
the	O
DTD	O
declared	O
in	O
the	O
document	O
)	O
.	O
</s>
<s>
When	O
a	O
named	O
entity	O
reference	O
is	O
then	O
encountered	O
in	O
the	O
rest	O
of	O
the	O
document	O
(	O
including	O
in	O
the	O
rest	O
of	O
the	O
DTD	O
)	O
,	O
and	O
if	O
this	O
entity	O
name	O
has	O
effectively	O
been	O
defined	O
as	O
a	O
parsed	B-Language
entity	O
,	O
the	O
reference	O
itself	O
is	O
replaced	O
immediately	O
by	O
the	O
textual	O
content	O
defined	O
in	O
the	O
parsed	B-Language
entity	O
,	O
and	O
the	O
parsing	B-Language
continues	O
within	O
this	O
replacement	O
text	O
.	O
</s>
<s>
Predefined	O
named	O
character	B-General_Concept
entities	I-General_Concept
are	O
similar	O
to	O
internal	O
entities	O
:	O
5	O
of	O
them	O
however	O
are	O
treated	O
specially	O
in	O
all	O
SGML	B-Language
,	O
HTML	B-Language
and	O
XML	B-Protocol
parsers	I-Protocol
.	O
</s>
<s>
These	O
entities	O
are	O
a	O
bit	O
different	O
from	O
normal	O
parsed	B-Language
entities	O
,	O
because	O
when	O
a	O
named	O
character	B-General_Concept
entity	I-General_Concept
reference	I-General_Concept
is	O
encountered	O
in	O
the	O
document	O
,	O
the	O
reference	O
is	O
also	O
replaced	O
immediately	O
by	O
the	O
character	O
content	O
defined	O
in	O
the	O
entity	O
,	O
but	O
the	O
parsing	B-Language
continues	O
after	O
the	O
replacement	O
text	O
,	O
which	O
is	O
immediately	O
inserted	O
literally	O
in	O
the	O
currently	O
parsed	B-Language
token	O
(	O
if	O
such	O
character	O
is	O
permitted	O
in	O
the	O
textual	O
value	O
of	O
that	O
token	O
)	O
.	O
</s>
<s>
This	O
allows	O
some	O
characters	O
that	O
are	O
needed	O
for	O
the	O
core	O
syntax	O
of	O
HTML	B-Language
or	O
XML	B-Protocol
themselves	O
to	O
be	O
escaped	O
from	O
their	O
special	O
syntactic	O
role	O
(	O
notably	O
"	O
&	O
"	O
which	O
is	O
reserved	O
for	O
beginning	O
entity	O
references	O
,	O
"	O
<" or ">	O
"	O
which	O
delimit	O
the	O
markup	O
tags	O
,	O
and	O
"	O
double	O
"	O
or	O
'	O
single	O
 '	O
quotation	O
marks	O
,	O
which	O
delimit	O
the	O
values	O
of	O
attributes	O
and	O
entity	O
definitions	O
)	O
.	O
</s>
<s>
Predefined	O
character	B-General_Concept
entities	I-General_Concept
also	O
include	O
numeric	O
character	O
references	O
that	O
are	O
handled	O
the	O
same	O
way	O
and	O
can	O
also	O
be	O
used	O
to	O
escape	O
the	O
characters	O
they	O
represent	O
,	O
or	O
to	O
bypass	O
limitations	O
in	O
the	O
character	O
repertoire	O
supported	O
by	O
the	O
document	O
encoding	O
.	O
</s>
<s>
In	O
basic	O
profiles	O
for	O
SGML	B-Language
or	O
in	O
HTML	B-Language
documents	O
,	O
the	O
declaration	O
of	O
internal	O
entities	O
is	O
not	O
possible	O
(	O
because	O
external	O
DTD	O
subsets	O
are	O
not	O
retrieved	O
,	O
and	O
internal	O
DTD	O
subsets	O
are	O
not	O
supported	O
in	O
these	O
basic	O
profiles	O
)	O
.	O
</s>
<s>
Instead	O
,	O
HTML	B-Language
standards	O
predefine	O
a	O
large	O
set	O
of	O
several	O
hundred	O
named	O
character	B-General_Concept
entities	I-General_Concept
,	O
which	O
can	O
still	O
be	O
handled	O
as	O
standard	O
parsed	B-Language
entities	O
defined	O
in	O
the	O
DTD	O
used	O
by	O
the	O
parser	O
.	O
</s>
<s>
They	O
are	O
just	O
declared	O
by	O
a	O
unique	O
name	O
in	O
the	O
document	O
,	O
and	O
defined	O
with	O
a	O
public	O
identifier	O
(	O
an	O
FPI	O
)	O
and/or	O
a	O
system	B-Language
identifier	I-Language
(	O
interpreted	O
as	O
an	O
URI	B-Protocol
)	O
specifying	O
where	O
the	O
source	O
of	O
their	O
content	O
.	O
</s>
<s>
parsed	B-Language
external	O
entities	O
(	O
most	O
often	O
defined	O
with	O
a	O
SYSTEM	B-Language
identifier	I-Language
indicating	O
the	O
URI	B-Protocol
of	O
their	O
content	O
)	O
that	O
are	O
not	O
associated	O
in	O
their	O
definition	O
to	O
a	O
named	O
annotation	O
,	O
in	O
which	O
case	O
validating	O
XML	B-Protocol
or	O
SGML	B-Language
parsers	B-Language
retrieve	O
their	O
contents	O
and	O
parse	B-Language
them	O
as	O
if	O
they	O
were	O
declared	O
as	O
internal	O
entities	O
(	O
the	O
external	O
entity	O
containing	O
their	O
effective	O
replacement	O
text	O
)	O
;	O
</s>
<s>
unparsed	O
external	O
entities	O
that	O
are	O
defined	O
and	O
associated	O
with	O
an	O
annotation	O
name	O
,	O
in	O
which	O
case	O
they	O
are	O
treated	O
as	O
opaque	O
references	O
and	O
signaled	O
as	O
such	O
to	O
the	O
application	O
using	O
the	O
SGML	B-Language
or	O
XML	B-Protocol
parser	I-Protocol
:	O
their	O
interpretation	O
,	O
retrieval	O
and	O
parsing	B-Language
is	O
left	O
to	O
the	O
application	O
,	O
according	O
to	O
the	O
types	O
of	O
annotations	O
it	O
supports	O
(	O
see	O
the	O
next	O
section	O
about	O
annotations	O
and	O
for	O
examples	O
of	O
unparsed	O
external	O
entities	O
)	O
.	O
</s>
<s>
External	O
entities	O
are	O
not	O
supported	O
in	O
basic	O
profiles	O
for	O
SGML	B-Language
or	O
in	O
HTML	B-Language
documents	O
,	O
but	O
are	O
valid	O
in	O
full	O
implementations	O
of	O
SGML	B-Language
and	O
in	O
XML	B-Protocol
1.0	O
or	O
1.1	O
(	O
including	O
XHTML	B-Language
and	O
SVG	O
,	O
even	O
if	O
they	O
are	O
not	O
strictly	O
needed	O
in	O
those	O
document	O
types	O
)	O
.	O
</s>
<s>
An	O
example	O
of	O
internal	O
entity	O
declarations	O
(	O
here	O
in	O
an	O
internal	O
DTD	O
subset	O
of	O
an	O
SGML	B-Language
document	O
)	O
is	O
:	O
</s>
<s>
Internal	O
entities	O
may	O
be	O
defined	O
in	O
any	O
order	O
,	O
as	O
long	O
as	O
they	O
are	O
not	O
referenced	O
and	O
parsed	B-Language
in	O
the	O
DTD	O
or	O
in	O
the	O
body	O
of	O
the	O
document	O
,	O
in	O
their	O
order	O
of	O
parsing	B-Language
:	O
it	O
is	O
valid	O
to	O
include	O
a	O
reference	O
to	O
a	O
still	O
undefined	O
entity	O
within	O
the	O
content	O
of	O
a	O
parsed	B-Language
entity	O
,	O
but	O
it	O
is	O
invalid	O
to	O
include	O
anywhere	O
else	O
any	O
named	O
entity	O
reference	O
before	O
this	O
entity	O
has	O
been	O
fully	O
defined	O
,	O
including	O
all	O
other	O
internal	O
entities	O
referenced	O
in	O
its	O
defined	O
content	O
(	O
this	O
also	O
prevents	O
circular	O
or	O
recursive	O
definitions	O
of	O
internal	O
entities	O
)	O
.	O
</s>
<s>
This	O
document	O
is	O
parsed	B-Language
as	O
if	O
it	O
was	O
:	O
</s>
<s>
Instead	O
,	O
it	O
is	O
replaced	O
only	O
when	O
the	O
"	O
signature	O
"	O
entity	O
reference	O
is	O
parsed	B-Language
within	O
the	O
content	O
of	O
the	O
"	O
sgml	B-Language
"	O
element	O
,	O
but	O
only	O
by	O
validating	O
parsers	B-Language
(	O
non-validating	O
parsers	B-Language
do	O
not	O
substitute	O
entity	O
references	O
occurring	O
within	O
contents	O
of	O
element	O
or	O
within	O
attribute	O
values	O
,	O
in	O
the	O
body	O
of	O
the	O
document	O
.	O
</s>
<s>
This	O
is	O
possible	O
because	O
the	O
replacement	O
text	O
specified	O
in	O
the	O
internal	O
entity	O
definitions	O
permits	O
a	O
distinction	O
between	O
parameter	O
entity	O
references	O
(	O
that	O
are	O
introduced	O
by	O
the	O
"	O
%	O
"	O
character	O
and	O
whose	O
replacement	O
applies	O
to	O
the	O
parsed	B-Language
DTD	O
contents	O
)	O
and	O
general	O
entity	O
references	O
(	O
that	O
are	O
introduced	O
by	O
the	O
"	O
&	O
"	O
character	O
and	O
whose	O
replacement	O
is	O
delayed	O
until	O
they	O
are	O
effectively	O
parsed	B-Language
and	O
validated	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
references	O
to	O
predefined	O
character	B-General_Concept
entities	I-General_Concept
are	O
substituted	O
wherever	O
they	O
occur	O
,	O
without	O
needing	O
a	O
validating	O
parser	O
(	O
they	O
are	O
only	O
introduced	O
by	O
the	O
"	O
&	O
"	O
character	O
)	O
.	O
</s>
<s>
Notations	O
are	O
used	O
in	O
SGML	B-Language
or	O
XML	B-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
notations	O
may	O
be	O
used	O
to	O
reference	O
non-XML	O
data	O
in	O
an	O
XML	B-Protocol
1.1	O
document	O
.	O
</s>
<s>
This	O
declares	O
the	O
MIME	B-Application
type	I-Application
of	O
external	O
images	O
with	O
this	O
type	O
,	O
and	O
associates	O
it	O
with	O
a	O
notation	O
name	O
"	O
type-image-svg	O
"	O
.	O
</s>
<s>
However	O
,	O
notation	O
names	O
usually	O
follow	O
a	O
naming	O
convention	O
that	O
is	O
specific	O
to	O
the	O
application	O
generating	O
or	O
using	O
the	O
notation	O
:	O
notations	O
are	O
interpreted	O
as	O
additional	O
meta-data	O
whose	O
effective	O
content	O
is	O
an	O
external	O
entity	O
and	O
either	O
a	O
PUBLIC	O
FPI	O
,	O
registered	O
in	O
the	O
catalogs	B-Language
used	O
by	O
XML	B-Protocol
or	O
SGML	B-Language
parsers	B-Language
,	O
or	O
a	O
SYSTEM	O
URI	B-Protocol
,	O
whose	O
interpretation	O
is	O
application	O
dependent	O
(	O
here	O
a	O
MIME	B-Application
type	I-Application
,	O
interpreted	O
as	O
a	O
relative	O
URI	B-Protocol
,	O
but	O
it	O
could	O
be	O
an	O
absolute	O
URI	B-Protocol
to	O
a	O
specific	O
renderer	O
,	O
or	O
a	O
URN	O
indicating	O
an	O
OS-specific	O
object	O
identifier	O
such	O
as	O
a	O
UUID	O
)	O
.	O
</s>
<s>
in	O
the	O
external	O
subset	O
as	O
well	O
as	O
the	O
internal	O
subset	O
,	O
at	O
least	O
for	O
conformance	O
with	O
XML	B-Protocol
.	O
</s>
<s>
Notations	O
can	O
be	O
associated	O
to	O
unparsed	O
external	O
entities	O
included	O
in	O
the	O
body	O
of	O
the	O
SGML	B-Language
or	O
XML	B-Protocol
document	I-Protocol
.	O
</s>
<s>
The	O
or	O
parameter	O
of	O
these	O
external	O
entities	O
specifies	O
the	O
FPI	O
and/or	O
the	O
URI	B-Protocol
where	O
the	O
unparsed	O
data	O
of	O
the	O
external	O
entity	O
is	O
located	O
,	O
and	O
the	O
additional	O
parameter	O
of	O
these	O
defined	O
entities	O
specifies	O
the	O
additional	O
notation	O
(	O
i.e.	O
,	O
effectively	O
the	O
MIME	B-Application
type	I-Application
here	O
)	O
.	O
</s>
<s>
Within	O
the	O
body	O
of	O
the	O
SGML	B-Language
document	O
,	O
these	O
referenced	O
external	O
entities	O
(	O
whose	O
name	O
is	O
specified	O
between	O
"	O
&	O
"	O
and	O
"	O
;	O
"	O
)	O
are	O
not	O
replaced	O
like	O
usual	O
named	O
entities	O
(	O
defined	O
with	O
a	O
CDATA	O
value	O
)	O
,	O
but	O
are	O
left	O
as	O
distinct	O
unparsed	O
tokens	O
that	O
may	O
be	O
used	O
either	O
as	O
the	O
value	O
of	O
an	O
element	O
attribute	O
(	O
like	O
above	O
)	O
or	O
within	O
the	O
element	O
contents	O
,	O
provided	O
that	O
either	O
the	O
DTD	O
allows	O
such	O
external	O
entities	O
in	O
the	O
declared	O
content	B-Application
type	I-Application
of	O
elements	O
or	O
in	O
the	O
declared	O
type	O
of	O
attributes	O
(	O
here	O
the	O
type	O
for	O
the	O
attribute	O
)	O
,	O
or	O
the	O
SGML	B-Language
parser	O
is	O
not	O
validating	O
the	O
content	O
.	O
</s>
<s>
The	O
example	O
above	O
shows	O
a	O
notation	O
named	O
"	O
type-image-svg	O
"	O
that	O
references	O
the	O
standard	O
public	O
FPI	O
and	O
the	O
system	B-Language
identifier	I-Language
(	O
the	O
standard	O
URI	B-Protocol
)	O
of	O
an	O
SVG	O
1.1	O
document	O
,	O
instead	O
of	O
specifying	O
just	O
a	O
system	B-Language
identifier	I-Language
as	O
in	O
the	O
first	O
example	O
(	O
which	O
was	O
a	O
relative	O
URI	B-Protocol
interpreted	O
locally	O
as	O
a	O
MIME	B-Application
type	I-Application
)	O
.	O
</s>
<s>
It	O
also	O
declares	O
another	O
notation	O
for	O
a	O
vendor-specific	O
application	O
,	O
to	O
annotate	O
the	O
"	O
sgml	B-Language
"	O
root	O
element	O
in	O
the	O
document	O
.	O
</s>
<s>
In	O
both	O
cases	O
,	O
the	O
declared	O
notation	O
named	O
is	O
used	O
directly	O
in	O
a	O
declared	O
"	O
type	O
"	O
attribute	O
,	O
whose	O
content	O
is	O
specified	O
in	O
the	O
DTD	O
with	O
the	O
"	O
NOTATION	O
"	O
attribute	O
type	O
(	O
this	O
"	O
type	O
"	O
attribute	O
is	O
declared	O
for	O
the	O
"	O
sgml	B-Language
"	O
element	O
,	O
as	O
well	O
as	O
for	O
the	O
"	O
img	O
"	O
element	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
"	O
title	O
"	O
attribute	O
of	O
the	O
"	O
img	O
"	O
element	O
specifies	O
the	O
internal	O
entity	O
"	O
example1SVGTitle	O
"	O
whose	O
declaration	O
that	O
does	O
not	O
define	O
an	O
annotation	O
,	O
so	O
it	O
is	O
parsed	B-Language
by	O
validating	O
parsers	B-Language
and	O
the	O
entity	O
replacement	O
text	O
is	O
"	O
Title	O
of	O
example1.svg	O
"	O
.	O
</s>
<s>
The	O
content	O
of	O
the	O
"	O
img	O
"	O
element	O
references	O
another	O
external	O
entity	O
"	O
example1SVG	O
"	O
whose	O
declaration	O
also	O
does	O
not	O
define	O
an	O
notation	O
,	O
so	O
it	O
is	O
also	O
parsed	B-Language
by	O
validating	O
parsers	B-Language
and	O
the	O
entity	O
replacement	O
text	O
is	O
located	O
by	O
its	O
defined	O
SYSTEM	B-Language
identifier	I-Language
"	O
example1.svg	O
"	O
(	O
also	O
interpreted	O
as	O
a	O
relative	O
URI	B-Protocol
)	O
.	O
</s>
<s>
The	O
difference	O
with	O
the	O
GIF	O
image	O
,	O
is	O
that	O
the	O
SVG	O
image	O
is	O
parsed	B-Language
within	O
the	O
SGML	B-Language
document	O
,	O
according	O
to	O
the	O
declarations	O
in	O
the	O
DTD	O
,	O
where	O
the	O
GIF	O
image	O
is	O
just	O
referenced	O
as	O
an	O
opaque	O
external	O
object	O
(	O
which	O
is	O
not	O
parsable	B-Language
with	O
SGML	B-Language
)	O
via	O
its	O
"	O
data	O
"	O
attribute	O
(	O
whose	O
value	O
type	O
is	O
an	O
opaque	O
ENTITY	O
)	O
.	O
</s>
<s>
Only	O
one	O
notation	O
name	O
may	O
be	O
specified	O
in	O
the	O
value	O
of	O
ENTITY	O
attributes	O
(	O
there	O
's	O
no	O
support	O
in	O
SGML	B-Language
,	O
XML	B-Protocol
1.0	O
or	O
XML	B-Protocol
1.1	O
for	O
multiple	O
notation	O
names	O
in	O
the	O
same	O
declared	O
external	O
ENTITY	O
,	O
so	O
separate	O
attributes	O
are	O
needed	O
)	O
.	O
</s>
<s>
Notations	O
are	O
also	O
completely	O
opaque	O
for	O
XML	B-Protocol
and	O
SGML	B-Language
parsers	B-Language
,	O
so	O
they	O
are	O
not	O
differentiated	O
by	O
the	O
type	O
of	O
the	O
external	O
entity	O
that	O
they	O
may	O
reference	O
(	O
for	O
these	O
parsers	B-Language
they	O
just	O
have	O
a	O
unique	O
name	O
associated	O
to	O
a	O
public	O
identifier	O
(	O
an	O
FPI	O
)	O
and/or	O
a	O
system	B-Language
identifier	I-Language
(	O
a	O
URI	B-Protocol
)	O
)	O
.	O
</s>
<s>
Some	O
applications	O
(	O
but	O
not	O
XML	B-Protocol
or	O
SGML	B-Language
parsers	B-Language
themselves	O
)	O
also	O
allow	O
referencing	O
notations	O
indirectly	O
by	O
naming	O
them	O
in	O
the	O
value	O
of	O
a	O
standard	O
CDATA	O
attribute	O
,	O
everywhere	O
a	O
URI	B-Protocol
can	O
be	O
specified	O
.	O
</s>
<s>
However	O
this	O
behaviour	O
is	O
application-specific	O
,	O
and	O
requires	O
that	O
the	O
application	O
maintains	O
a	O
catalog	O
of	O
known	O
URNs	O
to	O
resolve	O
them	O
into	O
the	O
notations	O
that	O
have	O
been	O
parsed	B-Language
in	O
a	O
standard	O
SGML	B-Language
or	O
XML	B-Protocol
parser	I-Protocol
.	O
</s>
<s>
This	O
use	O
allows	O
notations	O
to	O
be	O
defined	O
only	O
in	O
a	O
DTD	O
stored	O
as	O
an	O
external	O
entity	O
and	O
referenced	O
only	O
as	O
the	O
external	O
subset	O
of	O
documents	O
,	O
and	O
allows	O
these	O
documents	O
to	O
remain	O
compatible	O
with	O
validating	O
XML	B-Protocol
or	O
SGML	B-Language
parsers	B-Language
that	O
have	O
no	O
direct	O
support	O
for	O
notations	O
.	O
</s>
<s>
Notations	O
are	O
not	O
used	O
in	O
HTML	B-Language
,	O
or	O
in	O
basic	O
profiles	O
for	O
XHTML	B-Language
and	O
SVG	O
,	O
because	O
:	O
</s>
<s>
Standard	O
pseudo-attributes	O
in	O
XML	B-Protocol
and	O
XHTML	B-Language
(	O
such	O
as	O
xml:lang	B-Protocol
,	O
or	O
xmlns	B-Language
and	O
xmlns:*	O
for	O
namespace	O
declarations	O
)	O
.	O
</s>
<s>
Even	O
in	O
validating	O
SGML	B-Language
or	O
XML	B-Protocol
1.0	O
or	O
XML	B-Protocol
1.1	O
parsers	B-Language
,	O
the	O
external	O
entities	O
referenced	O
by	O
an	O
FPI	O
and/or	O
URI	B-Protocol
in	O
declared	O
notations	O
are	O
not	O
retrieved	O
automatically	O
by	O
the	O
parsers	B-Language
themselves	O
.	O
</s>
<s>
Instead	O
,	O
these	O
parsers	B-Language
just	O
provide	O
to	O
the	O
application	O
the	O
parsed	B-Language
FPI	O
and/or	O
URI	B-Protocol
associated	O
to	O
the	O
notations	O
found	O
in	O
the	O
parsed	B-Language
SGML	B-Language
or	O
XML	B-Protocol
document	I-Protocol
,	O
and	O
with	O
a	O
facility	O
for	O
a	O
dictionary	O
containing	O
all	O
notation	O
names	O
declared	O
in	O
the	O
DTD	O
;	O
these	O
validating	O
parsers	B-Language
also	O
check	O
the	O
uniqueness	O
of	O
notation	O
name	O
declarations	O
,	O
and	O
report	O
a	O
validation	O
error	O
if	O
some	O
notation	O
names	O
are	O
used	O
anywhere	O
in	O
the	O
DTD	O
or	O
in	O
the	O
document	O
body	O
but	O
not	O
declared	O
:	O
</s>
<s>
If	O
the	O
application	O
ca	O
n't	O
use	O
any	O
notation	O
(	O
or	O
if	O
their	O
FPI	O
and/or	O
URI	B-Protocol
are	O
unknown	O
or	O
not	O
supported	O
in	O
their	O
local	O
catalog	O
)	O
,	O
these	O
notations	O
may	O
be	O
either	O
ignored	O
silently	O
by	O
the	O
application	O
or	O
the	O
application	O
could	O
signal	O
an	O
error	O
.	O
</s>
<s>
Otherwise	O
,	O
the	O
applications	O
decide	O
themselves	O
how	O
to	O
interpret	O
them	O
,	O
then	O
if	O
the	O
external	O
entities	O
must	O
be	O
retrieved	O
and	O
then	O
parsed	B-Language
separately	O
.	O
</s>
<s>
Applications	O
may	O
then	O
signal	O
an	O
error	O
,	O
if	O
such	O
interpretation	O
,	O
retrieval	O
or	O
separate	O
parsing	B-Language
fails	O
.	O
</s>
<s>
The	O
XML	B-Language
DTD	I-Language
syntax	O
is	O
one	O
of	O
several	O
XML	B-Language
schema	I-Language
languages	O
.	O
</s>
<s>
However	O
,	O
many	O
of	O
the	O
schema	O
languages	O
do	O
not	O
fully	O
replace	O
the	O
XML	B-Language
DTD	I-Language
.	O
</s>
<s>
Notably	O
,	O
the	O
XML	B-Language
DTD	I-Language
allows	O
defining	O
entities	O
and	O
notations	O
that	O
have	O
no	O
direct	O
equivalents	O
in	O
DTD-less	O
XML	B-Protocol
(	O
because	O
internal	O
entities	O
and	O
parsable	B-Language
external	O
entities	O
are	O
not	O
part	O
of	O
XML	B-Language
schema	I-Language
languages	O
,	O
and	O
because	O
other	O
unparsed	O
external	O
entities	O
and	O
notations	O
have	O
no	O
simple	O
equivalent	O
mappings	O
in	O
most	O
XML	B-Language
schema	I-Language
languages	O
)	O
.	O
</s>
<s>
Most	O
XML	B-Language
schema	I-Language
languages	O
are	O
only	O
replacements	O
for	O
element	O
declarations	O
and	O
attribute	O
list	O
declarations	O
,	O
in	O
such	O
a	O
way	O
that	O
it	O
becomes	O
possible	O
to	O
parse	B-Language
XML	B-Protocol
documents	I-Protocol
with	O
non-validating	O
XML	B-Protocol
parsers	I-Protocol
(	O
if	O
the	O
only	O
purpose	O
of	O
the	O
external	O
DTD	O
subset	O
was	O
to	O
define	O
the	O
schema	O
)	O
.	O
</s>
<s>
In	O
addition	O
,	O
documents	O
for	O
these	O
XML	B-Language
schema	I-Language
languages	O
must	O
be	O
parsed	B-Language
separately	O
,	O
so	O
validating	O
the	O
schema	O
of	O
XML	B-Protocol
documents	I-Protocol
in	O
pure	O
standalone	O
mode	O
is	O
not	O
really	O
possible	O
with	O
these	O
languages	O
:	O
the	O
document	O
type	O
declaration	O
remains	O
necessary	O
for	O
at	O
least	O
identifying	O
(	O
with	O
a	O
XML	B-Language
Catalog	I-Language
)	O
the	O
schema	O
used	O
in	O
the	O
parsed	B-Language
XML	B-Protocol
document	I-Protocol
and	O
that	O
is	O
validated	O
in	O
another	O
language	O
.	O
</s>
<s>
A	O
common	O
misconception	O
holds	O
that	O
a	O
non-validating	O
XML	B-Protocol
parser	I-Protocol
does	O
not	O
have	O
to	O
read	O
document	O
type	O
declarations	O
,	O
when	O
in	O
fact	O
,	O
the	O
document	O
type	O
declarations	O
must	O
still	O
be	O
scanned	O
for	O
correct	O
syntax	O
as	O
well	O
as	O
validity	O
of	O
declarations	O
,	O
and	O
the	O
parser	O
must	O
still	O
parse	B-Language
all	O
entity	O
declarations	O
in	O
the	O
internal	O
subset	O
,	O
and	O
substitute	O
the	O
replacement	O
texts	O
of	O
internal	O
entities	O
occurring	O
anywhere	O
in	O
the	O
document	O
type	O
declaration	O
or	O
in	O
the	O
document	O
body	O
.	O
</s>
<s>
A	O
non-validating	O
parser	O
may	O
,	O
however	O
,	O
elect	O
not	O
to	O
read	O
parsable	B-Language
external	O
entities	O
(	O
including	O
the	O
external	O
subset	O
)	O
,	O
and	O
does	O
not	O
have	O
to	O
honor	O
the	O
content	O
model	O
restrictions	O
defined	O
in	O
element	O
declarations	O
and	O
in	O
attribute	O
list	O
declarations	O
.	O
</s>
<s>
If	O
the	O
XML	B-Protocol
document	I-Protocol
depends	O
on	O
parsable	B-Language
external	O
entities	O
(	O
including	O
the	O
specified	O
external	O
subset	O
,	O
or	O
parsable	B-Language
external	O
entities	O
declared	O
in	O
the	O
internal	O
subset	O
)	O
,	O
it	O
should	O
assert	O
standalone	O
=	O
"	O
no	O
"	O
in	O
its	O
XML	B-Protocol
declaration	O
.	O
</s>
<s>
The	O
validating	O
DTD	O
may	O
be	O
identified	O
by	O
using	O
XML	B-Language
Catalogs	I-Language
to	O
retrieve	O
its	O
specified	O
external	O
subset	O
.	O
</s>
<s>
In	O
the	O
example	O
below	O
,	O
the	O
XML	B-Protocol
document	I-Protocol
is	O
declared	O
with	O
standalone	O
=	O
"	O
no	O
"	O
because	O
it	O
has	O
an	O
external	O
subset	O
in	O
its	O
document	O
type	O
declaration	O
:	O
</s>
<s>
If	O
the	O
XML	B-Protocol
document	I-Protocol
type	O
declaration	O
includes	O
any	O
SYSTEM	B-Language
identifier	I-Language
for	O
the	O
external	O
subset	O
,	O
it	O
ca	O
n't	O
be	O
safely	O
processed	O
as	O
standalone	O
:	O
the	O
URI	B-Protocol
should	O
be	O
retrieved	O
,	O
otherwise	O
there	O
may	O
be	O
unknown	O
named	O
character	B-General_Concept
entities	I-General_Concept
whose	O
definition	O
may	O
be	O
needed	O
to	O
correctly	O
parse	B-Language
the	O
effective	O
XML	B-Protocol
syntax	O
in	O
the	O
internal	O
subset	O
or	O
in	O
the	O
document	O
body	O
(	O
the	O
XML	B-Protocol
syntax	O
parsing	B-Language
is	O
normally	O
performed	O
after	O
the	O
substitution	O
of	O
all	O
named	O
entities	O
,	O
excluding	O
the	O
five	O
entities	O
that	O
are	O
predefined	O
in	O
XML	B-Protocol
and	O
that	O
are	O
implicitly	O
substituted	O
after	O
parsing	B-Language
the	O
XML	B-Protocol
document	I-Protocol
into	O
lexical	O
tokens	O
)	O
.	O
</s>
<s>
If	O
it	O
just	O
includes	O
any	O
PUBLIC	O
identifier	O
,	O
it	O
may	O
be	O
processed	O
as	O
standalone	O
,	O
if	O
the	O
XML	B-Protocol
processor	O
knows	O
this	O
PUBLIC	O
identifier	O
in	O
its	O
local	O
catalog	O
from	O
where	O
it	O
can	O
retrieve	O
an	O
associated	O
DTD	O
entity	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
very	O
simple	O
external	O
XML	B-Language
DTD	I-Language
to	O
describe	O
the	O
schema	O
of	O
a	O
list	O
of	O
persons	O
might	O
consist	O
of	O
:	O
</s>
<s>
name	O
is	O
a	O
valid	O
element	O
name	O
,	O
and	O
an	O
instance	O
of	O
such	O
an	O
element	O
contains	O
"	O
parsed	B-Language
character	O
data	O
"	O
(	O
#PCDATA	O
)	O
.	O
</s>
<s>
birthdate	O
is	O
a	O
valid	O
element	O
name	O
,	O
and	O
an	O
instance	O
of	O
such	O
an	O
element	O
contains	O
parsed	B-Language
character	O
data	O
.	O
</s>
<s>
gender	O
is	O
a	O
valid	O
element	O
name	O
,	O
and	O
an	O
instance	O
of	O
such	O
an	O
element	O
contains	O
parsed	B-Language
character	O
data	O
.	O
</s>
<s>
socialsecuritynumber	O
is	O
a	O
valid	O
element	O
name	O
,	O
and	O
an	O
instance	O
of	O
such	O
an	O
element	O
contains	O
parsed	B-Language
character	O
data	O
.	O
</s>
<s>
An	O
example	O
of	O
an	O
XML	B-Protocol
file	O
that	O
uses	O
and	O
conforms	O
to	O
this	O
DTD	O
follows	O
.	O
</s>
<s>
The	O
DTD	O
is	O
referenced	O
here	O
as	O
an	O
external	O
subset	O
,	O
via	O
the	O
SYSTEM	O
specifier	O
and	O
a	O
URI	B-Protocol
.	O
</s>
<s>
It	O
assumes	O
that	O
we	O
can	O
identify	O
the	O
DTD	O
with	O
the	O
relative	O
URI	B-Protocol
reference	O
"	O
example.dtd	O
"	O
;	O
the	O
"	O
people_list	O
"	O
after	O
"	O
!	O
DOCTYPE	O
"	O
tells	O
us	O
that	O
the	O
root	O
tags	O
,	O
or	O
the	O
first	O
element	O
defined	O
in	O
the	O
DTD	O
,	O
is	O
called	O
"	O
people_list	O
"	O
:	O
</s>
<s>
One	O
can	O
render	O
this	O
in	O
an	O
XML-enabled	O
browser	B-Application
(	O
such	O
as	O
Internet	B-Application
Explorer	I-Application
or	O
Mozilla	B-Application
Firefox	I-Application
)	O
by	O
pasting	O
and	O
saving	O
the	O
DTD	O
component	O
above	O
to	O
a	O
text	O
file	O
named	O
example.dtd	O
and	O
the	O
XML	B-Protocol
file	O
to	O
a	O
differently-named	O
text	O
file	O
,	O
and	O
opening	O
the	O
XML	B-Protocol
file	O
with	O
the	O
browser	B-Application
.	O
</s>
<s>
However	O
,	O
many	O
browsers	B-Application
do	O
not	O
check	O
that	O
an	O
XML	B-Protocol
document	I-Protocol
confirms	O
to	O
the	O
rules	O
in	O
the	O
DTD	O
;	O
they	O
are	O
only	O
required	O
to	O
check	O
that	O
the	O
DTD	O
is	O
syntactically	O
correct	O
.	O
</s>
<s>
The	O
same	O
DTD	O
can	O
also	O
be	O
embedded	O
directly	O
in	O
the	O
XML	B-Protocol
document	I-Protocol
itself	O
as	O
an	O
internal	O
subset	O
,	O
by	O
encasing	O
it	O
within	O
[	O
square	O
brackets ]	O
in	O
the	O
document	O
type	O
declaration	O
,	O
in	O
which	O
case	O
the	O
document	O
no	O
longer	O
depends	O
on	O
external	O
entities	O
and	O
can	O
be	O
processed	O
in	O
standalone	O
mode	O
:	O
</s>
<s>
XML	B-Language
Schema	I-Language
,	O
also	O
referred	O
to	O
as	O
XML	B-Language
Schema	I-Language
Definition	O
(	O
XSD	O
)	O
,	O
has	O
achieved	O
Recommendation	O
status	O
within	O
the	O
W3C	O
,	O
and	O
is	O
popular	O
for	O
"	O
data	O
oriented	O
"	O
(	O
that	O
is	O
,	O
transactional	O
non-publishing	O
)	O
XML	B-Protocol
use	O
because	O
of	O
its	O
stronger	O
typing	O
and	O
easier	O
round-tripping	O
to	O
Java	O
declarations	O
.	O
</s>
<s>
An	O
XML	B-Language
Schema	I-Language
Definition	O
is	O
itself	O
an	O
XML	B-Protocol
document	I-Protocol
while	O
a	O
DTD	O
is	O
not	O
.	O
</s>
<s>
RELAX	B-Language
NG	I-Language
,	O
which	O
is	O
also	O
a	O
part	O
of	O
DSDL	B-Language
,	O
is	O
an	O
ISO	O
international	O
standard	O
.	O
</s>
<s>
An	O
XML	B-Language
DTD	I-Language
can	O
be	O
used	O
to	O
create	O
a	O
denial	O
of	O
service	O
(	O
DoS	O
)	O
attack	O
by	O
defining	O
nested	O
entities	O
that	O
expand	O
exponentially	O
,	O
or	O
by	O
sending	O
the	O
XML	B-Protocol
parser	I-Protocol
to	O
an	O
external	O
resource	O
that	O
never	O
returns	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
.NET	O
Framework	O
provides	O
a	O
property	O
that	O
allows	O
prohibiting	O
or	O
skipping	O
DTD	O
parsing	B-Language
,	O
and	O
recent	O
versions	O
of	O
Microsoft	O
Office	O
applications	O
(	O
Microsoft	O
Office	O
2010	O
and	O
higher	O
)	O
refuse	O
to	O
open	O
XML	B-Protocol
files	O
that	O
contain	O
DTD	O
declarations	O
.	O
</s>
