CSS: The Definitive Guide, 3rd Edition (7 page)

The last type of attribute selector, the
particular attribute selector, is easier to show than it is to describe. Consider the
following rule:

*[lang|="en"] {color: white;}

This rule will select any element whose
lang
attribute is equal to
en
or begins with
en-
. Therefore, the first three elements in the
following example markup would be selected, but the last two would not:

Hello!
Greetings!

G'day!

Bonjour!

Jrooana!

In general, the form
[att|="val"]
can be used
for any attribute and its values. Let's say you have a series of figures in an HTML
document, each of which has a filename like
figure-1.gif
and
figure-3.jpg
. You can match all of these images using the
following selector:

img[src|="figure"] {border: 1px solid gray;}

The most common use for this type of attribute selector is to match language
values, as demonstrated later in this chapter.

To understand the
relationship between selectors and documents, you need to once again examine how
documents are structured. Consider this very simple HTML document:

Meerkat Central

Welcome to Meerkat Central, the best meerkat web site
on the entire Internet!


We offer:

Detailed information on how to adopt a meerkat

Tips for living with a meerkat

Fun things to do with a meerkat, including:

Playing fetch

Digging for food

Hide and seek





...and so much more!



Questions? Contact us!


        	
			  
        	

Much of the power of CSS is based on the
parent-child
relationship
of elements. HTML
documents (actually, most structured documents of any kind) are based on a hierarchy
of
elements, which is visible in the "tree" view of the document (see
Figure 2-14
). In this hierarchy, each
element fits somewhere into the overall structure of the document. Every element in
the document is either the
parent
or the
child
of another element, and it's often both.

Figure 2-14. A document tree structure

An element is said to be the parent of another element if it appears directly
above that element in the document hierarchy. For example, in
Figure 2-14
, the first
p
element is parent to the
em
and
strong
elements, while
strong
is parent to an
anchor
element, which is itself parent to another
em
element. Conversely, an element is the child of another element if
it is directly beneath the other element. Thus, the anchor element in
Figure 2-14
is a child of the
strong
element, which is in turn child to the
p
element, and so on.

The terms parent and child are specific applications of the terms
ancestor
and
descendant
. There is a
difference between them: in the tree view, if an element is exactly one level above
another, then they have a parent-child relationship. If the path from one element to
another continues through two or more levels, the elements have an
ancestor-descendant relationship, but not a parent-child relationship. (Of course, a
child is also a descendant, and a parent is an ancestor.) In
Figure 2-14
, the first
ul
element is parent to two
li
elements, but the first
ul
is also
the ancestor of every element descended from its
li
element, all the way down to the most deeply nested
li
elements.

Also, in
Figure 2-14
, there is an
anchor that is a child of
strong
, but also a
descendant of
paragraph
,
body
, and
html
elements. The
body
element is an ancestor of everything that
the browser will display by default, and the
html
element is ancestor to the entire document. For this reason, the
html
element is also called the
root
element
.

The first benefit of understanding this model is the
ability to define
descendant selectors
(also known as
contextual selectors
).
Defining descendant selectors is the act of creating
rules that operate in certain structural circumstances but not others. As an example,
let's say you want to style only those
em
elements
that are descended from
h1
elements. You could put
a
class
attribute on every
em
element found within an
h1
, but that's almost as time-consuming as using the
font
tag. It's obviously far more efficient to declare
rules that match only
em
elements that are found
inside
h1
elements.

To do so, write the following:

h1 em {color: gray;}

This rule will make gray any text in an
em
element that is the descendant of an
h1
element.
Other
em
text, such as that found in a paragraph
or a block quote, will not be selected by this rule.
Figure 2-15
makes this clear.

Figure 2-15. Selecting an element based on its context

In a descendant selector, the selector side of a rule is composed of two or more
space-separated selectors. The space between the selectors is an example of a
combinator
. Each space combinator can be translated as "found
within," "which is part of," or "that is a descendant of," but only if you read the
selector right to left. Thus,
h1 em
can be
translated as, "Any
em
element that is a
descendant of an
h1
element." (To read the
selector left to right, you might phrase it something like, "Any
h1
that contains an
em
will have the following styles applied to the
em
.")

You aren't limited to two selectors, of course. For example:

ul ol ul em {color: gray;}

In this case, as
Figure 2-16
shows,
any emphasized text that is part of an unordered list that is part of an ordered list
that is itself part of an unordered list (yes, this is correct) will be gray. This is
obviously a very specific selection criterion.

Figure 2-16. A very specific descendant selector

Descendant selectors can be extremely powerful. They make possible what could
never be done in HTML—at least not without oodles of
font
tags. Let's consider a common example. Assume you have a document
with a sidebar and a main area. The sidebar has a blue background, the main area has
a white background, and both areas include lists of links. You can't set all links to
be blue because they'd be impossible to read in the sidebar.

The solution: descendant selectors. In this case, you give the table cell that
contains your sidebar a class of
sidebar
, and
assign the main area a class of
main
. Then, you
write styles like this:

td.sidebar {background: blue;}
td.main {background: white;}
td.sidebar a:link {color: white;}
td.main a:link {color: blue;}

Figure 2-17
shows the result.

Figure 2-17. Using descendant selectors to apply different styles to the same type of
element

Here's another example: let's say that you want gray to be the text color of any
b
(boldface) element that is part of a
blockquote
, and also for any bold text that is found in
a normal paragraph:

blockquote b, p b {color: gray;}

The result is that the text within
b
elements
that are descended from paragraphs or block quotes will be gray.

One overlooked aspect of descendant selectors is that the degree of separation
between two elements can be practically infinite. For example, if you write
ul em
, that syntax will select any
em
element descended from a
ul
element, no matter how deeply nested the
em
may be. Thus,
ul em
would select the
em
element in the following markup:

List item 1

List item 1-1

List item 1-2

List item 1-3

List item 1-3-1

List item 1-3-2

List item 1-3-3


List item 1-4

In some cases, you don't want to select an
arbitrarily descended element; rather, you want to narrow your range to select an
element that is a child of another element. You might, for example, want to select a
strong
element only if it is a child (as
opposed to a descendant) of an
h1
element. To do
this, you use the child combinator,
which is the
greater-than symbol (
>
):

h1 > strong {color: red;}

This rule will make red the
strong
element
shown in the first
h1
below, but not the second:

This is very important.
This is really very important.

Read right to left, the selector
h1 > strong
translates as "selects any
strong
element that is
a child of an
h1
element." The child combinator is
optionally surrounded by whitespace. Thus,
h1 > strong,
h1> strong
, and
h1>strong
are
all equivalent. You can use or omit whitespace as you wish.

When viewing the document as a tree structure, it's easy to see that a child
selector restricts its matches to elements that are directly connected in the tree.
Figure 2-18
shows part of a document
tree.

Figure 2-18. A document tree fragment

In this tree fragment, you can easily pick out parent-child relationships. For
example, the
a
element is parent to the
strong
, but it is child to the
p
element. You could match elements in this fragment with the selectors
p > a
and
a >
strong
, but not
p > strong
, since
the
strong
is a descendant of the
p
but not its child.

You can also combine descendant and child combinations in the same selector. Thus,
table.summary td > p
will select any
p
element that is a child of a
td
element that is itself descended from a
table
element that has a
class
attribute containing the word
summary
.