Discussions of web typography are usually about the frustrating inconsistencies and paucity of decent fonts likely to be available on a user’s machine. But when a client recently made a very sobering comment concerning cross-platform display, I got a rather rude reminder of how easily it is to forget more fundamental issues. Namely, anti-aliasing (or ‘smoothing’ in Microsoft-speak).
According to Wikipedia*, Windows XP is still the predominant OS (with about two-thirds of the market), and it does not anti-alias fonts by default (though the feature can be enabled in the OS settings). Windows Vista does smooth type by default, although it only accounts for about 20% of the overall market (despite being available for more than two years now). Macs account for about 5% of the market.
Meanwhile, 48% of web users are using IE7 & IE8, which anti-alias type irrespective of the OS settings. IE6 does not though, and accounts for another 19% of the market. (Firefox, with about 20% market share, and other browsers such as Safari, obey OS smoothing settings.)
So what does all this mean? That only about half the user population definitively views anti-aliased type. Which means that—duh—the other half might not. There’s no way of knowing how many XP users have turned on their smoothing, but its probably safer to assume that many haven’t. I turned mine on a long time ago and never thought about it again, but it’s not necessarily the kind of thing most people would think to do and it’s also not the easiest thing to locate amongst the Windows Control Panels either.
For further reference, here are three composite screenshots: one showing Windows core fonts (plus Mac-only Courier) with ClearType smoothing enabled (a 96dpi PNG from IE7); one without (also a 96dpi PNG, this time from IE6); and one showing the same font set in Firefox on Mac OS X (at 72dpi). There are obviously too many subtle details to point out, but one key thing to keep an eye on is how differently the smaller bold weights display with and without smoothing enabled, especially in the italic styles.
Update: there’s some worthwhile explanation of smoothing behaviour to be found on this Typophile thread.