Its not really a difficult thing to do so much as you (I don't think anyway) can't do it in HTML. So you simply need to have a method to process it.
With PHP, and using sunfighter's xslt saved as steamfeed.xsl, I would simply do as so:
$sFeed = 'http://api.steampowered.com/ISteamUserStats/GetGlobalAchievementPercentagesForApp/v0002/?gameid=221380&format=xml';
$dom = new DOMDocument('1.0', 'utf-8');
$xsl = new XSLTProcessor();
Simple as that.