W3C CSS2 Cascading Style Sheets, level 2 - instructional media + ...

This text will be beneath everything.
This text will underlay text1, but overlay the butterfly image
This example demonstrates the notion of transparency. The default behavior of
a box is to allow boxes behind it to be visible through transparent areas in its
content. In the example, each box transparently overlays the boxes below it. This
behavior can be overridden by using one of the existing background properties
[p. 188] .
9.10 Text direction: the ’direction’ and
’unicode-bidi’ properties
The characters in certain scripts are written from right to left. In some documents,
in particular those written with the Arabic or Hebrew script, and in some
mixed-language contexts, text in a single (visually displayed) block may appear
with mixed directionality. This phenomenon is called bidirectionality, or "bidi" for
short.
The Unicode standard ([UNICODE], section 3.11) defines a complex algorithm
for determining the proper directionality of text. The algorithm consists of an
implicit part based on character properties, as well as explicit controls for embeddings
and overrides. CSS2 relies on this algorithm to achieve proper bidirectional
rendering. The ’direction’ and ’unicode-bidi’ properties allow authors to specify
how the elements and attributes of a document language map to this algorithm.
If a document contains right-to-left characters, and if the user agent displays
these characters (with appropriate glyphs, not arbitrary substitutes such as a
question mark, a hex code, a black box, etc.), the user agent must apply the bidirectional
algorithm. This seemingly one-sided requirement reflects the fact that,
although not every Hebrew or Arabic document contains mixed-directionality text,
such documents are much more likely to contain left-to-right text (e.g., numbers,
text from other languages) than are documents written in left-to-right languages.
Because the directionality of a text depends on the structure and semantics of
the document language, these properties should in most cases be used only by
designers of document type descriptions (DTDs), or authors of special documents.
If a default style sheet specifies these properties, authors and users
should not specify rules to override them. A typical exception would be to override
bidi behavior in a user agent if that user agent transliterates Yiddish (usually
written with Hebrew letters) to Latin letters at the user’s request.
The HTML 4.0 specification ([HTML40], section 8.2) defines bidirectionality
behavior for HTML elements. Conforming [p. 32] HTML user agents may therefore
ignore the ’direction’ and ’unicode-bidi’ properties in author and user style
sheets. The style sheet rules that would achieve the bidi behavior specified in
[HTML40] are given in the sample style sheet [p. 292] . The HTML 4.0 specifica-
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