Ville Viklund wrote:
Is there a good reason to use this kind of selector "#header ul li a
{...", instead of just "#header li a"? Is it considered best practice,
or why this is so commonly used?
I would venture a guess that it is commonly used because of a fairly
widespread misunderstanding of exactly how contextual selectors work.
If one consults (e.g.) Htmlhelp.com, an early reference site for
HTML & CSS, one is told :
Contextual selectors are merely strings of two or more simple selectors
separated by white space. These selectors can be assigned normal properties
and, due to the rules of cascading order, they will take precedence over simple
selectors. For example, the contextual selector in
P EM { background: yellow }
is P EM. This rule says that emphasized text within a paragraph should have a
yellow background; emphasized text in a heading would be unaffected.
Note "emphasized text within a paragraph"; nowhere does this
suggest that the rule might apply to (e.g.) "emphasized text
within spanned element within a paragraph", so the naïve user
might believe that in order to achieve the latter, he/she
should write "P SPAN EM".
One could even argue that the W3 prose is not as helpful
as it might be, although the spec. is unambiguous if
one reads it properly. For example, at :
http://www.w3.org/TR/REC-CSS1/#contextual-selectors
one can read (for H1 EM) "In the example above, the search
pattern matches if 'EM' is a descendant of 'H1',
i.e. if 'EM' is inside an 'H1' element."
Note "is inside an 'H1' element", which again may suggest
to the naïve reader that it must be /directly/ inside.
I really think it is as simple as that : a widespread
misunderstanding.
Philip Taylor
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