<s>
A	O
style	B-Language
sheet	I-Language
language	I-Language
,	O
or	O
style	B-Language
language	I-Language
,	O
is	O
a	O
computer	B-Language
language	I-Language
that	O
expresses	O
the	O
presentation	O
of	O
structured	O
documents	O
.	O
</s>
<s>
One	O
modern	O
style	B-Language
sheet	I-Language
language	I-Language
with	O
widespread	O
use	O
is	O
Cascading	B-Language
Style	I-Language
Sheets	I-Language
(	O
CSS	B-Language
)	O
,	O
which	O
is	O
used	O
to	O
style	O
documents	O
written	O
in	O
HTML	B-Language
,	O
XHTML	B-Language
,	O
SVG	B-Application
,	O
XUL	B-Application
,	O
and	O
other	O
markup	O
languages	O
.	O
</s>
<s>
In	O
electronic	O
publishing	O
,	O
style	B-Language
sheet	I-Language
languages	I-Language
are	O
mostly	O
used	O
in	O
the	O
context	O
of	O
visual	O
presentation	O
rather	O
than	O
spelling	O
and	O
punctuation	O
.	O
</s>
<s>
All	O
style	B-Language
sheet	I-Language
languages	I-Language
offer	O
functionality	O
in	O
these	O
areas	O
:	O
</s>
<s>
SyntaxA	O
style	B-Language
sheet	I-Language
language	I-Language
needs	O
a	O
syntax	O
in	O
order	O
to	O
be	O
expressed	O
in	O
a	O
machine-readable	O
manner	O
.	O
</s>
<s>
For	O
example	O
,	O
here	O
is	O
a	O
simple	O
style	O
sheet	O
written	O
in	O
the	O
CSS	B-Language
syntax:This	O
says	O
that	O
headings	O
on	O
level	O
1	O
should	O
be	O
displayed	O
in	O
a	O
font	O
size	O
of	O
1.5	O
times	O
the	O
font	O
size	O
of	O
the	O
surrounding	O
text	O
.	O
</s>
<s>
Properties	O
All	O
style	B-Language
sheet	I-Language
languages	I-Language
have	O
some	O
concept	O
of	O
properties	O
that	O
can	O
be	O
given	O
values	O
to	O
change	O
one	O
aspect	O
of	O
rendering	O
an	O
element	O
.	O
</s>
<s>
The	O
"	O
font-size	O
"	O
property	O
of	O
CSS	B-Language
is	O
used	O
in	O
the	O
above	O
example	O
.	O
</s>
<s>
Common	O
style	B-Language
sheet	I-Language
languages	I-Language
typically	O
have	O
around	O
50	O
properties	O
to	O
describe	O
the	O
presentation	O
of	O
documents	O
.	O
</s>
<s>
The	O
"	O
em	O
"	O
value	O
in	O
CSS	B-Language
refers	O
to	O
the	O
font	O
size	O
of	O
the	O
surrounding	O
text	O
.	O
</s>
<s>
Common	O
style	B-Language
sheet	I-Language
languages	I-Language
have	O
around	O
ten	O
different	O
units	O
.	O
</s>
<s>
Value	O
propagation	O
mechanism	O
To	O
avoid	O
having	O
to	O
specify	O
explicitly	O
all	O
values	O
for	O
all	O
properties	O
on	O
all	O
elements	O
,	O
style	B-Language
sheet	I-Language
languages	I-Language
have	O
mechanisms	O
to	O
propagate	O
values	O
automatically	O
.	O
</s>
<s>
Formatting	O
model	O
All	O
style	B-Language
sheet	I-Language
languages	I-Language
support	O
some	O
kind	O
of	O
formatting	O
model	O
.	O
</s>
<s>
Most	O
style	B-Language
sheet	I-Language
languages	I-Language
have	O
a	O
visual	O
formatting	O
model	O
that	O
describes	O
,	O
in	O
some	O
detail	O
,	O
how	O
text	O
and	O
other	O
content	O
is	O
laid	O
out	O
in	O
the	O
final	O
presentation	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
CSS	B-Language
formatting	O
model	O
specifies	O
that	O
block-level	O
elements	O
(	O
of	O
which	O
"	O
h1	O
"	O
is	O
an	O
example	O
)	O
extend	O
to	O
fill	O
the	O
width	O
of	O
the	O
parent	O
element	O
.	O
</s>
<s>
Some	O
style	B-Language
sheet	I-Language
languages	I-Language
also	O
have	O
an	O
aural	O
formatting	O
model	O
.	O
</s>
