Приложение C: Кодирование
(Это приложение является
информативным, а не формальным)
HTML documents may contain any of the about 30,000
different characters defined by Unicode. Many documents only need
a few hundred. Many fonts also only contain just a few hundred
glyphs. In combination with section
5.2, this appendix explains how the
characters in the document and the glyphs in a font are matched.
The content of an HTML document is a sequence of characters
and markup. To send it "over the wire", it is encoded
as a sequence of bytes, using one of several possible encodings.
The HTML document has to be decoded to find the characters. For
example, in Western Europe it is customary to use the byte 224
for an a-with-grave-accent (a), but in Hebrew, it is more common
to use 224 for an Aleph. In Japanese, the meaning of a byte
usually depends on the bytes that preceded it. In some encodings,
one character is encoded as two (or more) bytes.
The UA knows how to decode the bytes by looking at the
"charset" parameter in the HTTP header. Typical
encodings (charset values) are "ASCII" (for English),
"ISO-8859-1" (for Western Europe),
"ISO-8859-8" (for Hebrew), "Shift-JIS" (for
Japanese).
HTML [2][4], allows some 30,000
different characters, namely those defined by Unicode. Not many
documents will use that many different characters, and choosing
the right encoding will usually ensure that the document only
needs one byte per character. Occasional characters outside the
encoded range can still be entered as numerical character
references: 'Π' will always mean the Greek uppercase Pi,
no matter what encoding was used. Note that this entails that UAs
have to be prepared for any Unicode character, even if they only
handle a few encodings.
A font doesn't contain characters,
it contains pictures of characters, known as glyphs.
The glyphs, in the form of outlines or bitmaps, constitute a
particular representation of a character. Either explicitly or
implicitly, each font has a table associated with it, the font
encoding table, that tells for each glyph
what character it is a representation for. In Type 1 fonts, the
table is referred to as an encoding vector.
In fact, many fonts contain several glyphs for the
same character. Which of those glyphs should be used depends
either on the rules of the language, or on the preference of the
designer.
In Arabic, for example, all letters have four
different shapes, depending on whether the letter is used at the
start of a word, in the middle, at the end, or in isolation. It
is the same character in all cases, and thus there is only one
character in the HTML document, but when printed, it looks
differently each time.
There are also fonts that leave it to the graphic
designer to choose from among various alternative shapes
provided. Unfortunately, CSS1 doesn't yet provide the means to
select those alternatives. Currently, it is always the default
shape that is chosen from such fonts.
To deal with the problem that a single font may not be
enough to display all the characters in a document, or even a
single element, CSS1 allows the use of font
sets.
A font set in CSS1 is a list of fonts, all of the same
style and size, that are tried in sequence to see if they contain
a glyph for a certain character. An element that contains English
text mixed with mathematical symbols may need a font set of two
fonts, one containing letters and digits, the other containing
mathematical symbols. See section
5.2 for a detailed description of the
selection mechanism for font sets.
Here is an example of a font set suitable for a text
that is expected to contain text with Latin characters, Japanese
characters, and mathematical symbols:
BODY { font-family: Baskerville, Mincho, Symbol, serif }
The characters available in the Baskerville font (a
font with only Latin characters) will be taken from that font,
Japanese will be taken from Mincho, and the mathematical symbols
will come from Symbol. Any other characters will (hopefully) come
from the generic font family 'serif'. The 'serif' font family
will also be used if one or more of the other fonts is
unavailable.
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