Another recommendation regarding formatting.
41. [MEASURES] – Do not use pixel measures and do not use absolute units in markup language attribute values and style sheet property values.
I discussed absolute vs. relative units in part 2 in relation to scrolling, but I’ll define this issue again here and expand on it.
From my consulting and training experience, I find that authors usually define the size of fonts and spacing in points, such as 10pt for Normal text. Two reasons for this:
• Many help authoring tools, like RoboHelp, use points as the default so we’ve just gotten used to it.
• For authors who come from hard-copy, especially Word, points is the default and an easily visualized unit – 1 point = 1/72”, so 12 points = 1/6”…, for example
But when we put content online, absolute units like points cause trouble for two reasons.
• Different display technologies may render points differently. For example, 10 pt is standard for Normal text on PC displays but too small on Macs because of the different technologies. So if you’re creating online material that might be read on PCs and Macs, you’d need two CSSs, one for each, with two different sizes for Normal text.
This isn’t a problem if you create online content for local use on one platform, such as HTML Help on PCs. But if you have to convert that content to a format like WebHelp that might sit on a server and be accessed by users with different platforms, like PCs and Macs, this does become a problem.
• Browsers can’t resize point-based text.
Going relative fixes these problems easily, once you get used to thinking in this new way. Relative units replace points, pixels, inches, etc., with units like % or ems. The benefit of relative units is that they automatically adapt, under the browser’s control, to the space on a given display.
For example, a graphic that’s 180 pixels wide might be fine on a standard display but too wide for a mobile device, so the device adds a horizontal scroll bar. Horizontal scrolling isn’t evil, but it does reduce usability, especially if you have to scroll vertically and horizontally, so you want to avoid it where possible.
Setting the width to 100% fixes the problem. This says that the graphic should display at 100% of the available space and leaves it up to the browser to figure out what that is.
This approach isn’t perfect. It’s possible to apply relative size to a graphic which displays it correctly but makes it too small to read. Or it may not display correctly in certain cases, such as within table cells in ePub format running in the Adobe Digital Editions (ADE) viewer. Still trying to track this one down…
What relative units can you use? There are several, including %, ems, and exes. The % is relative to the size of Normal style at 100% on any browser. So, for example, setting h1 style to 150% says that all heading 1 text should be half again as large as Normal text on each browser. (This isn’t exactly the case, but it’s close enough to get a mental picture.) The em is based on the baseline height of the m on the browser. The ex is similar to the em but based on the x.
Of these three, the most widely supported (a totally unscientific statement based solely on experience) are % and em. Some people prefer the em. Others, myself included, prefer the % because it’s easier to picture mentally. Pick the one that makes the most sense for you and is easiest to use.
Next, keeping style sheets small and using terse, efficient markup…
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