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Приложение B: Грамматика CSS1. Спецификация CSS1


Дата публикации: 05-01-2011

(Это приложение является формальным)

The minimal CSS (i.e., any version of CSS) grammar that all implementations need to support is defined in section 7. The grammar below defines a much smaller language, a language that defines the syntax of CSS1.

It is in some sense, however, still a superset of CSS1: there are additional semantic constraints not expressed in this grammar. A conforming UA must also adhere to the forward-compatible parsing rules (section 7.1), the property and value notation (section 5) and the unit notation (section 6). In addition, HTML imposes restrictions, e.g., on the possible values of the CLASS attribute.

The grammar below is LL(1) (but note that most UA's should not use it directly, since it doesn't express the parsing conventions, only the CSS1 syntax). The format of the productions is optimized for human consumption and some shorthand notation beyond yacc [15] is used:

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.

Character encoding

 

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 (à), 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.

Font encoding

 

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.

Font sets

 

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.

Appendix D: Gamma correction

 

(This appendix is informative, not normative)

See the

Gamma Tutorial

in the PNG specification [12] if you aren't already familiar with gamma issues.

 

In the computation, UAs displaying on a CRT may assume an ideal CRT and ignore any effects on apparent gamma caused by dithering. That means the minimal handling they need to do on current platforms is:

PC using MS-Windows
none
Unix using X11
none
Mac using QuickDraw
apply gamma 1.39 [13] (ColorSync-savvy applications may simply pass the sRGB ICC profile [14] to ColorSync to perform correct color correction)
SGI using X
apply the gamma value from /etc/config/system.glGammaVal (the default value being 1.70; applications running on Irix 6.2 or above may simply pass the sRGB ICC profile to the color management system)
NeXT using NeXTStep
apply gamma 2.22

"Applying gamma" means that each of the three R, G and B must be converted to R'=Rgamma, G'=Ggamma, G'=Bgamma, before handing to the OS.

This may rapidly be done by building a 256-element lookup table once per browser invocation thus:

  for i := 0 to 255 do
    raw := i / 255;
    corr := pow (raw, gamma);
    table[i] := trunc (0.5 + corr * 255.0)
  end

which then avoids any need to do transcendental math per color attribute, far less per pixel.

Appendix E: The applicability and extensibility of CSS1

 

(This appendix is informative, not normative)

The goal of the work on CSS1 has been to create a simple style sheet mechanism for HTML documents. The current specification is a balance between the simplicity needed to realize style sheets on the web, and pressure from authors for richer visual control. CSS1 offers:

  • visual markup replacement: HTML extensions, e.g. "CENTER", "FONT" and "SPACER", are easily replaced with CSS1 style sheets.
  • nicer markup: instead of using "FONT" elements to achieve the popular small-caps style, one declaration in the style sheet is sufficient. Compare the visual markup:
      <H1>H<FONT SIZE=-1>EADLINE</FONT></H1>
    

    with the style sheet:

      H1 { font-style: small-caps }
      <H1>Headline</H1>
    
  • various integration levels: CSS1 style rules can be fetched from external style sheets, included in the 'STYLE' element, or put into 'STYLE' attributes. The latter option offers easy transition from HTML extensions.
  • new effects: some new visual effects have been added to offer users new toys. The typographical pseudo-elements and the extra values on the background property fall into this category.
  • scalability: CSS1 will be useful on equipment ranging from text terminals to high-resolution color workstations. Authors can write one style sheet and be reasonably sure that the intended style will come across in the best possible manner.

CSS1 does not offer:

  • per pixel control: CSS1 values simplicity over level of control, and although the combination of background images and styled HTML is powerful, control to the pixel level is not possible.
  • author control: the author cannot enforce the use of a certain sheet, only suggest
  • a layout language: CSS1 does not offer multiple columns with text-flow, overlapping frames etc.
  • a rich query language on the parse tree: CSS1 can only look for ancestor elements in the parse tree, while other style sheet languages (e.g. DSSSL [6]) offers a full query language.

We expect to see extensions of CSS in several directions:

  • paper: better support for printing HTML documents
  • support for non-visual media: work is in the process to add a list of properties and corresponding values to support speech and braille output
  • color names: the currently supported list may be extended
  • fonts: more precise font specification systems are expected to complement existing CSS1 font properties.
  • values, properties: we expect vendors to propose extensions to the CSS1 set of values and properties. Extending in this direction is trivial for the specification, but interoperability between different UAs is a concern
  • layout language: support for two-dimensional layout in the tradition of desktop publishing packages.
  • other DTDs: CSS1 has some HTML-specific parts (e.g. the special status of the 'CLASS' and 'ID' attributes) but should easily be extended to apply to other DTDs as well.

We do not expect CSS to evolve into:

  • a programming language

 

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