<s>
OPML	B-Application
(	O
Outline	B-Application
Processor	I-Application
Markup	I-Application
Language	I-Application
)	O
is	O
an	O
XML	B-Protocol
format	O
for	O
outlines	O
(	O
defined	O
as	O
"	O
a	O
tree	O
,	O
where	O
each	O
node	O
contains	O
a	O
set	O
of	O
named	O
attributes	O
with	O
string	O
values	O
"	O
)	O
.	O
</s>
<s>
Originally	O
developed	O
by	O
UserLand	O
as	O
a	O
native	O
file	O
format	O
for	O
the	O
outliner	B-Application
application	O
in	O
its	O
Radio	B-Protocol
UserLand	I-Protocol
product	O
,	O
it	O
has	O
since	O
been	O
adopted	O
for	O
other	O
uses	O
,	O
the	O
most	O
common	O
being	O
to	O
exchange	O
lists	O
of	O
web	B-General_Concept
feeds	I-General_Concept
between	O
web	B-General_Concept
feed	I-General_Concept
aggregators	O
.	O
</s>
<s>
The	O
OPML	B-Application
specification	O
defines	O
an	O
outline	O
as	O
a	O
hierarchical	O
,	O
ordered	O
list	O
of	O
arbitrary	O
elements	O
.	O
</s>
<s>
Support	O
for	O
importing	O
and	O
exporting	O
RSS	O
feed	O
lists	O
in	O
OPML	B-Application
format	O
is	O
available	O
in	O
Mozilla	B-Application
Thunderbird	I-Application
,	O
and	O
many	O
other	O
RSS	B-Application
reader	I-Application
web	O
sites	O
and	O
applications	O
.	O
</s>
<s>
The	O
XML	B-Protocol
elements	O
in	O
an	O
OPML	B-Application
document	O
are	O
:	O
</s>
<s>
An	O
OPML	B-Application
processor	O
may	O
ignore	O
all	O
the	O
head	O
sub-elements	O
.	O
</s>
<s>
Due	O
to	O
the	O
arbitrary	O
nature	O
of	O
the	O
"	O
type	O
"	O
attribute	O
,	O
and	O
the	O
acceptance	O
of	O
arbitrary	O
attributes	O
on	O
"	O
outline	O
"	O
elements	O
,	O
interoperability	O
of	O
OPML	B-Application
documents	O
relies	O
almost	O
entirely	O
on	O
the	O
undocumented	O
conventions	O
of	O
content	O
producers	O
.	O
</s>
<s>
MIME	O
types	O
are	O
n't	O
clear	O
;	O
text/xml	B-Protocol
usually	O
merely	O
identifies	O
the	O
document	O
as	O
having	O
XML	B-Protocol
content	O
,	O
while	O
text/x	O
-opml	O
does	O
not	O
specify	O
that	O
the	O
document	O
is	O
XML	B-Protocol
since	O
it	O
lacks	O
the	O
+xml	O
suffix	O
(	O
see	O
XML	B-Protocol
and	I-Protocol
MIME	I-Protocol
)	O
.	O
</s>
<s>
Katy	O
Ginger	O
from	O
DLESE	O
has	O
published	O
for	O
validating	O
OPML	B-Application
2.0	O
.	O
</s>
<s>
Alternative	O
specifications	O
have	O
been	O
proposed	O
,	O
notably	O
these	O
include	O
OML	B-General_Concept
,	O
XOXO	B-General_Concept
and	O
XBEL	B-Language
.	O
</s>
