<s>
Browser	B-Protocol
sniffing	I-Protocol
(	O
also	O
known	O
as	O
browser	B-Protocol
detection	I-Protocol
)	O
is	O
a	O
set	O
of	O
techniques	O
used	O
in	O
websites	O
and	O
web	B-Application
applications	I-Application
in	O
order	O
to	O
determine	O
the	O
web	B-Application
browser	I-Application
a	O
visitor	O
is	O
using	O
,	O
and	O
to	O
serve	O
browser-appropriate	O
content	O
to	O
the	O
visitor	O
.	O
</s>
<s>
It	O
is	O
also	O
used	O
to	O
detect	O
mobile	B-Application
browsers	I-Application
and	O
send	O
them	O
mobile-optimized	O
websites	O
.	O
</s>
<s>
This	O
practice	O
is	O
sometimes	O
used	O
to	O
circumvent	O
incompatibilities	O
between	O
browsers	B-Application
due	O
to	O
misinterpretation	O
of	O
HTML	B-Language
,	O
Cascading	B-Language
Style	I-Language
Sheets	I-Language
(	O
CSS	B-Language
)	O
,	O
or	O
the	O
Document	B-General_Concept
Object	I-General_Concept
Model	I-General_Concept
(	O
DOM	B-General_Concept
)	O
.	O
</s>
<s>
While	O
the	O
World	O
Wide	O
Web	O
Consortium	O
maintains	O
up-to-date	O
central	O
versions	O
of	O
some	O
of	O
the	O
most	O
important	O
Web	O
standards	O
in	O
the	O
form	O
of	O
recommendations	O
,	O
in	O
practice	O
no	O
software	O
developer	O
has	O
designed	O
a	O
browser	B-Application
which	O
adheres	O
exactly	O
to	O
these	O
standards	O
;	O
implementation	O
of	O
other	O
standards	O
and	O
protocols	O
,	O
such	O
as	O
SVG	B-Application
and	O
XMLHttpRequest	B-Protocol
,	O
varies	O
as	O
well	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
different	O
browsers	B-Application
display	O
the	O
same	O
page	O
differently	O
,	O
and	O
so	O
browser	B-Protocol
sniffing	I-Protocol
was	O
developed	O
to	O
detect	O
the	O
web	B-Application
browser	I-Application
in	O
order	O
to	O
help	O
ensure	O
consistent	O
display	O
of	O
content	O
.	O
</s>
<s>
Web	O
pages	O
can	O
use	O
programming	O
languages	O
such	O
as	O
JavaScript	B-Language
which	O
are	O
interpreted	O
by	O
the	O
user	B-Protocol
agent	I-Protocol
,	O
with	O
results	O
sent	O
to	O
the	O
web	B-Application
server	I-Application
.	O
</s>
<s>
This	O
code	O
is	O
run	O
by	O
the	O
client	B-Protocol
computer	I-Protocol
,	O
and	O
the	O
results	O
are	O
used	O
by	O
other	O
code	O
to	O
make	O
necessary	O
adjustments	O
on	O
client-side	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
the	O
client	B-Protocol
computer	I-Protocol
is	O
asked	O
to	O
determine	O
whether	O
the	O
browser	B-Application
can	O
use	O
a	O
feature	O
called	O
ActiveX	B-Application
.	O
</s>
<s>
Since	O
this	O
feature	O
was	O
proprietary	O
to	O
Microsoft	O
,	O
a	O
positive	O
result	O
will	O
indicate	O
that	O
the	O
client	B-Protocol
may	O
be	O
running	O
Microsoft	O
's	O
Internet	B-Application
Explorer	I-Application
.	O
</s>
<s>
This	O
is	O
no	O
longer	O
a	O
reliable	O
indicator	O
since	O
Microsoft	O
's	O
open-source	O
release	O
of	O
the	O
ActiveX	B-Application
code	O
,	O
however	O
,	O
meaning	O
that	O
it	O
can	O
be	O
used	O
by	O
any	O
browser	B-Application
.	O
</s>
<s>
The	O
web	B-Application
server	I-Application
communicates	O
with	O
the	O
client	B-Protocol
using	O
a	O
communication	O
protocol	O
known	O
as	O
HTTP	B-Protocol
,	O
or	O
Hypertext	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
,	O
which	O
specifies	O
that	O
the	O
client	B-Protocol
send	O
the	O
server	B-Application
information	O
about	O
the	O
browser	B-Application
being	O
used	O
to	O
view	O
the	O
web	O
site	O
in	O
a	O
user	B-Protocol
agent	I-Protocol
string	O
.	O
</s>
<s>
Extensive	O
browser	B-Application
techniques	O
enable	O
persistent	O
user	O
tracking	O
even	O
when	O
users	O
try	O
to	O
stay	O
anonymous	O
.	O
</s>
<s>
See	O
device	B-Protocol
fingerprint	I-Protocol
for	O
more	O
details	O
on	O
browser	B-Protocol
fingerprinting	I-Protocol
,	O
a	O
relatively	O
new	O
,	O
extensive	O
browser	B-Protocol
sniffing	I-Protocol
on	O
steroids	O
technique	O
.	O
</s>
<s>
Many	O
websites	O
use	O
browser	B-Protocol
sniffing	I-Protocol
to	O
determine	O
whether	O
a	O
visitor	O
's	O
browser	B-Application
is	O
unable	O
to	O
use	O
certain	O
features	O
(	O
such	O
as	O
JavaScript	B-Language
,	O
DHTML	B-General_Concept
,	O
ActiveX	B-Application
,	O
or	O
cascading	B-Language
style	I-Language
sheets	I-Language
)	O
,	O
and	O
display	O
an	O
error	O
page	O
if	O
a	O
certain	O
browser	B-Application
is	O
not	O
used	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
virtually	O
impossible	O
to	O
account	O
for	O
the	O
tremendous	O
variety	O
of	O
browsers	B-Application
available	O
to	O
users	O
.	O
</s>
<s>
Generally	O
,	O
a	O
web	O
designer	O
using	O
browser	B-Protocol
sniffing	I-Protocol
to	O
determine	O
what	O
kind	O
of	O
page	O
to	O
present	O
will	O
test	O
for	O
the	O
three	O
or	O
four	O
most	O
popular	O
browsers	B-Application
,	O
and	O
provide	O
content	O
tailored	O
to	O
each	O
of	O
these	O
.	O
</s>
<s>
If	O
a	O
user	O
is	O
employing	O
a	O
user	B-Protocol
agent	I-Protocol
not	O
tested	O
for	O
,	O
there	O
is	O
no	O
guarantee	O
that	O
a	O
usable	O
page	O
will	O
be	O
served	O
;	O
thus	O
,	O
the	O
user	O
may	O
be	O
forced	O
either	O
to	O
change	O
browsers	B-Application
or	O
to	O
avoid	O
the	O
page	O
.	O
</s>
<s>
The	O
World	O
Wide	O
Web	O
Consortium	O
,	O
which	O
sets	O
standards	O
for	O
the	O
construction	O
of	O
web	O
pages	O
,	O
recommends	O
that	O
web	O
sites	O
be	O
designed	O
in	O
accordance	O
with	O
its	O
standards	O
,	O
and	O
be	O
arranged	O
to	O
"	O
fail	O
gracefully	O
"	O
when	O
presented	O
to	O
a	O
browser	B-Application
which	O
cannot	O
deal	O
with	O
a	O
particular	O
standard	O
.	O
</s>
