[CSS21] Clarification re Default Media Type

In the CR draft for CSS2.1 section on media types, it explicitly states:

Media types are mutually exclusive in the sense that a user
agent can only support one media type when rendering a
document. However, user agents may use different media types
on different canvases. For example, a document may
(simultaneously) be shown in 'screen' mode on one canvas and
'print' mode on another canvas.

Note that a multimodal media type is still only one media
type. The 'tv' media type, for example, is a multimodal
media type that renders both visually and aurally to a
single canvas.

This undermines the argument that the @media all rule applies when no
specific media type is specified; however, it has been made plain -- at
least in email form -- from the editors of the CSS 2.1 draft -- that the
@media all rule DOES apply when no specific media type is specified;


as Al Gilman, WAI PF's chair, pointed out during this exchange, in the
Visual Formatting Model section, the display property is explicitly
defined as pertaining to all media types, and yet, in the definition of
the none value for the display property is found the following:


    This value causes an element to generate no boxes in the
    formatting structure (i.e., the element has no effect on
    layout). Descendant elements do not generate any boxes
    either; this behavior cannot be overridden by setting
    the 'display' property on the descendants.

    Please note that a display of 'none' does not create an
    invisible box; it creates no box at all. CSS includes
    mechanisms that enable an element to generate boxes in
    the formatting structure that affect formatting but are
    not visible themselves. Please consult the section on
    visibility for details.

This could be misread to imply: "when the display property's value is
none, it is not included in the graphical formatting structure (that
is, layout), but should still be available as a kind of generated
content, which occupies no part of the visual canvas, but which is
rendered in the aural canvas."

Other than in the definition of the display property, our reviewer
couldn't find another instance in the draft that indicates that it
applies to all media types.

Assuming that you *do* mean display='none' to apply to all media, the
Protocols & Formats working group requests that clarifying wording be
added to the CSS2.1 Technical Recommendation where appropriate, so as
to faithfully implement the consequences of the default media type
being 'all' when no media type is defined. It needs to be clear
whether display=none means nothing is presented to the user, in any
medium, not only a graphical medium involving layout boxes.

/chair, PFWG

Received on Thursday, 20 December 2007 18:22:42 UTC