- From: Wendy A Chisholm <wendy@w3.org>
- Date: Wed, 07 Jan 2004 15:39:08 -0500
- To: gdeering@acslink.net.au, Jens Meiert <jens.meiert@erde3.com>
- Cc: w3c-wai-gl@w3.org
Geoff, You wrote: >What is the Accessibility / Usability rule of thumb for using absolute and >relative units? Can someone please point me to a very good explanation to >this (with examples), because it seems to me that something like the above >example is much better being addressed in relative units (maybe I'm wrong). >I use relative as much as possible. For what cases should absolute be >used in place of relative? The two relevant sections of the CSS Techniques doc are: 1. 2.9 Using relative units of measure <http://www.w3.org/WAI/GL/WCAG20/WD-WCAG20-CSS-TECHS-20031219.html#units-that-change> 2. 2.10 Using absolute units of measure <http://www.w3.org/WAI/GL/WCAG20/WD-WCAG20-CSS-TECHS-20031219.html#units-for-static> Section 2.9 quotes Charles Munat, " Think about what needs to change in size and what doesn't. Raster images, for example, have fixed width and heights. We specify them in the img tag in pixels...Similarly, if I just want to move the text away from the border of the page, setting the padding on the body to 5 px shouldn't be a problem. Text, on the other hand, should always be free to change size (a good reason not to use text in images). But when my text doubles in size, do I need the gutter next to it to double in size as well? Probably not. In fact, doubling the gutter might even decrease the usability of the page." > In the case of this CSS Tech doc, which encourages absolute for > margins, in pixels, does that only apply to media for screen only? What > do you do for handhelds, etc. Doesn't there need to be a distinction here? Good questions. CSS Techniques is primarily written from the point of view of HTML. We need to generalize it for other technologies and/or move some of the existing CSS Techniques into a CSS section of the HTML Techniques. > What is wrong with > >BODY { margin-left: 0.5em; margin-right: 0.5em} > >instead of >BODY { margin-left: 15px; margin-right: 15px} > >Isn't the relative one more flexible for user and device? Relative is more flexible, but as the font size increases so does the value of "em" and therefore the margin increases and you end up with a lot of white space. This can squeeze the width of the text into a narrow column making it difficult to read. In "CSS Design: Creating Custom Corners & Borders" by Søren Madsen [1] font-size *and* margins are in ems, width in percent, but it is padding that is defined in pixels. This works well in Mozilla 1.5 when I increase the font size to 200%, except that the white border in the middle column ends about half-way down the page. Therefore, we ought to test the following "rule of thumb:" if you specify margin alone, use an absolute unit otherwise you can use relative units for margin, but use absolute for padding. I'm looking for more articles and examples, please send related links to the list. --wendy [1] http://www.alistapart.com/articles/customcorners/ -- wendy a chisholm world wide web consortium web accessibility initiative http://www.w3.org/WAI/ /--
Received on Wednesday, 7 January 2004 15:42:41 UTC