Formatting Unicode Text; the W3C Approach
Stephen Zilles - Adobe Systems, Inc.
The purpose of my paper is to explain the features that were added to CSS and XSL to better support positioning and alignment of internationalized text. CSS (levels 1 and 2) was defined only for a single alphabetic baseline and the horizontal writing mode. Recent work on XSL and CSS3 has extended the CSS model for both horizontal and vertical writing modes and for scripts that do not use the alphabetic baseline.
The extended CSS/XSL model is based on the Open Type font model. This model posits a set of alignment baselines for different scripts, e.g., alphabetic, ideographic and hanging scripts. This allows characters in a given script, but presented in different font sizes, to be aligned on the baseline natural to that script. Distinguishing alignment baselines means that the "vertical-align" property of CSS needs to be subdivided into different roles. The basic idea is that some point on an inline box is aligned to some baseline of its parent. Therefore, properties to designate a "dominant-baseline", an alignment point and an "alignment-baseline" are introduced. Since baseline-shift moves all the alignment points, uniformly, it is also handled as separate property.
The extension to vertical writing modes is accomplished by adding a "writing-mode" property that specifies both the inline-progression-direction, the direction in which characters are placed and the block-progression-direction, the direction in which blocks and lines are stacked; for example, Japanese has the top to bottom, right to left writing mode. In addition, a number of properties are expressible in a form that is relative to the writing-mode; for example, padding-before.
The paper will illustrate how these facilities work together to provide a powerful solution for formatting text with multiple scripts and writing-modes. Examples will be taken from the XSL and CSS specifications.
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