This is still a work-in-progress. The gist of the content is written, but I'm still adding details and examples.
To be fair, the same flexibility offered to horizontal scripts in
vertical layout should be defined for vertical scripts in horizontal
layout as well. This requires defining a
property that handles the layout of vertical scripts in a horizontal-flow
Glyph orientation scheme to use in horizontal text. Only affects layout of non-horizontal scripts. Can take the following values
leftif the block's directionality has a
ltrhorizontal component; same as
The values for
text-orientation-horizontal are direction-based
rather than glyph-orientation-based as for
This asymmetry is because of
(It is an attempt to design each property,
text-orientation-horizontal, to be better suited to its own
Traditional Mongolian is strongly vertical, but fragments of it may be
set horizontally when they appear in the context of a horizontal script
block. As most horizontal scripts are left-to-right, Mongolian's designated
horizontal directionality in Unicode is left-to-right. It is not, however,
a strongly left-to-right script, and an Arabic typesetter could decide to
set it right-to-left, in line with the Arabic. Avoiding bidirectionality
makes a mixed text easier to read, so using
text-orientation-horizontal: context would make inline
Mongolian adapt to the direction of the primary script.
Most systems don't support vertical text, especially not left-to-right columns. Mongolian computer typesetting systems often work around this problem by laying out the text in horizontal lines from right to left, using an upside-down font, then rotating the page 90° to get vertical text. This technique wouldn't work quite well in a Unicode-based system, however, because a) Mongolian is designated left-to-right b) Unicode character shaping assumes this c) it requires using an upside-down font.
text-orientation-horizontal: right is defined,
the layout system would take care of shaping the characters properly,
ordering them from right to left, and flipping the glyphs. A system that
only supports horizontal text layout could thus take standard Unicode
text and, using regular Monoglian fonts, print out correctly-set
Mongolian. Because it does not use font, encoding, or directionality
hacks, the same text can be transferred without change to a different
Unicode-based typesetting system that has full vertical text support
and still render correctly.
Historically, Chinese was not written horizontally, only vertically, in columns ordered right to left. Over the course of the twentieth century, Chinese (and Japanese and Korean) became bi-orientational, acquiring a strong left-to-right directionality in horizontal text. However, there still remains some use of what appears to be right-to-left Chinese in single-line banners and the like. This is generally viewed as a logical extension of vertical text layout: it is Chinese in vertical columns, ordered right to left—except the columns are only one character tall.
The effect can be achieved, without using a right-to-left override,
by stripping the Chinese text of its horizontal directionality with
a top-to-bottom (
ttb) override and styling it with
text-orientation-horizontal: context or
text-orientation-horizontal: right. Because it still
retains the property of being a vertical script, it will still render
as top-to-bottom text when placed in a vertical layout.