When high-DPI is turned on WebKit continues to display very small text. WebKit should instead set the initial zoom level based on the system DPI setting. This will cause the text/images in the WebView to be the right size compared to the rest of the system.
<rdar://problem/6900322>
CSS system fonts are already scaled by the system based on the DPI setting. If a page uses CSS system fonts, and WebKit sets the initial zoom level based on the DPI setting, then won't those fonts get double-scaled?