Intent to implement and ship: vertical text support (CSS writing-mode and related properties)

[Jonathan Kew]

We are working on support for

v
e
r
t
i
c
a
l

text in Gecko. This will allow web content (and eventually complete web
apps, if desired) to use vertical writing, as traditionally used in East
Asia.

In addition to the CSS writing-mode property, this will involve support
for text-orientation (which determines whether glyphs within vertical
text remain "upright" as if in horizontal text, or are rotated 90° to
render "sideways"), and for logical versions of many properties that
refer to physical directions (such as margin-left, padding-top,
border-bottom, min-width, max-height, etc. etc.), so that stylesheets --
in particular, the UA's built-in stylesheets -- can be written in a form
that works for both horizontal and vertical content.

The top-level meta bug covering this work is
https://bugzilla.mozilla.org/show_bug.cgi?id=145503.

The major standards relevant to this work are:
http://dev.w3.org/csswg/css-writing-modes/
http://dev.w3.org/csswg/css-logical-props/

This will be part of the standard Gecko engine, available to HTML
content across all products and platforms. (At this time, we are not
planning to support vertical writing modes for XUL.)

Our intention is to implement and release support for vertical text and
layout in stages: initially, vertical mode will be supported for simple
blocks and inline elements, including styling and editing text as
expected. Some of the simpler form controls such as checkboxes, radio
buttons, input fields and textareas should also be usable in the first
release. However, more complex controls and layout constructs will not
be vertical-enabled until later releases. Thus, the initial release
should be adequate to support pages that want to display straightforward
blocks of text using vertical writing (e.g. a news site, blog, or ebook
reader).

There is already some vertical writing mode support in webkit, blink and
IE, and tests are being developed (e.g. see
http://www.gtalbot.org/BrowserBugsSection/CSS3WritingModes/). Current
implementations are prefixed, and appear to be somewhat incomplete
and/or based on earlier drafts of the specs, but there seems to be a
good prospect of browsers converging as the specifications become more
stable.

A good deal of the layout work to support writing modes has landed over
the past year. It is controlled by the boolean pref
"layout.css.vertical-text.enabled", which is currently false by default.

We intend to enable this feature by toggling the vertical-text pref to
true in https://bugzilla.mozilla.org/show_bug.cgi?id=1099032 once the
code is sufficiently stable. This may be in Gecko 38, if the remaining
dependencies can be resolved.

JK