Edit comment LC-1050 for Accessibility Guidelines Working Group

Quick access to

Previous: LC-521 Next: LC-656

Comment LC-1050
Commenter: Marco Bertoni <mbertoni@webaccessibile.org>

Resolution status:

I think that we need some clarifications about the terms "relative", "absolute" and "relative positioning", expecially when they are used regarding both font sizing units and CSS positioning rules.

In the "Comparison of WCAG 1.0 checkpoints to WCAG 2.0" [1] appendix we can read:

In General (Priority 2):
3.4: Use relative rather than absolute units in markup language attribute values and style sheet property values.

that is is related to:

WCAG 2.0 Success Criteria:
This maps to an advisory technique (Using readable fonts) for Guideline 1.4.

Guideline 1.4 refer to visual and audio contrast [2] and I've not yet found, even in the "Techniques for WCAG 2.0", the advisory technique mentioned (Using readable fonts).

Even in the "CSS Techniques for WCAG 2.0" draft [2] - in which is correctly introduced the idea of "Using em or percent for properties that need to change" instead of "Use relative rather than absolute units..." - the corresponding guideline mentioned is "1.3" ...that refer to the separation of structure and presentation.

However, as Gregg Vanderheiden told: "Most advisory techniques have not been written yet".

Having said that, I must point up that using terms like "relative units", "absolute units" or "relative positioning" maybe somehow confusing.

I'll start from the following assumptions:

1) The general accessibility principle in question is that users (expecially partially sighted people) must be able to enlarge fonts using their visual UA tools.

2) Microsoft Internet Explorer 6 for Windows - and all the previous versions - is not able to enlarge text if it is sized in pixels. But pixels are "relative" lenght units (they are relative to the viewing device resolution) [4].

3) On the other hand there are some CSS absolute-size values that IE/WIN can perfectly enlarge: the absolute-size keywords (x-small, small, medium etc.) [5].

So, it is really confusing to advise CSS designers that they should use relative length units to size fonts.

CSS Techniques correctly tell us to "Use "em" or % for properties that need to change". But we should make another stride to avoid to mention a specific unit identifier: e.g. here in Italy, in our recent accessibility law [6], we have introduced a requirement (Requirement 12) that says: "The presentation and textual content of a page must be able to be adapted to the dimensions of the browser window used by the user, without superimposing objects present or loss of information such as to render the content incomprehensible, including in the case of resizing, enlargement or reduction of the display area or characters in relation to the predefined values of these parameters.". In other words, with this approach we say that the designer must size characters with a unit of his choice that guarantees the enlargement of text in every UA. This is expecially useful to ensure forward and backward compatibility. We cannot forget that, at the time of this writing, IE 6 cover almost all the market.

Cynthia Shelly suggested the term "scalable" to describe this text resizing function.

Summarizing, there are relative and absolute CSS units that are scalable (also in IE6/WIN):

1) for fonts: em, percentages and absolute-keywords (e.g. medium, small, x-small etc.)
2) for containers: percentages and em.

But we have to think about the near future when IE/WIN will be able to make pixels scalable. So we definitively should talk about *functions* rather than units, and I agree with Cynthia that "scalable" is the right term to use.

Therefore I suggest to tell designers something like "Use scalable units for CSS properties that need to change: font-size and line-height" rather than "Use these units...".

Directly following that we must also clarify the use of the phrase "relative positioning": when a CSS designer hear that words he think about a CSS rule like this: #box { position: relative; } [7]. But, in itself, a positioning scheme isn't related to accessibility.

When you talk about relative positioning I think that you mean a concept like "use liquid design" rather than a specific CSS positioning scheme. It is wrong to advise designers to use a specific positioning scheme. Instead, i.e., we should tell them that a liquid design is more accessible than a fixed design, no matter which technique they use to obtain that. Obviously we must first justify this suggestion!.

These terminological problems are really serious: I've hardly discussed, here in Italy, with tricky designers that keep on using their beloved pixels for text sizing because these are relative units as checkpoint 3.4 of WCAG 1.0 suggest (no matter if partially sighted people using IE/WIN cannot resize them).

Marco Bertoni
Referente Regionale IWA Liguria
International Webmasters Association ITALIA
Fax Verde: 800 12.64.96 - Web Site: http://www.iwa-italy.org

[1] http://www.w3.org/TR/WCAG20/appendixD.html
[2] http://www.w3.org/TR/WCAG20/guidelines.html#visual-audio-contrast
[3] http://www.w3.org/TR/WCAG20-CSS-TECHS/#syntax-data-types
[4] http://www.w3.org/TR/CSS21/syndata.html#length-units
[5] http://www.w3.org/TR/CSS21/fonts.html#font-size-props
[6] http://www.pubbliaccesso.it/normative/DM080705-A-en.htm
[7] http://www.w3.org/TR/CSS21/visuren.html#relative-positioning
(space separated ids)
(Please make sure the resolution is adapted for public consumption)

Developed and maintained by Dominique Hazaël-Massieux (dom@w3.org).
$Id: 1050.html,v 1.1 2017/08/11 06:42:13 dom Exp $
Please send bug reports and request for enhancements to w3t-sys.org