It's also the case that the default stylesheets for the web are terrible, for historical reasons. If the default stylesheets were reasonable, then at least 80%

It's also the case that the default stylesheets for the web are terrible, for historical reasons. If the default stylesheets were reasonable, then at least 80% of the time you could just use semantic HTML5 and be sure that your presentation was at least OK in terms of visual hierarchy, accessibility, etc. As things actually stand, you need at minimum "100 bytes of CSS to Look Great Everywhere", and if that doesn't meet your needs, a classless stylesheet.