The v0.8.0 release did not include the SVG images which are necessary for rendering wide and stretchy accents as well as \overbrace and \underbrace. This release corrects that.
Note that if you're including copies of KaTeX in your web application, you should now include the dist/images directory in addition to the usual the dist/fonts directory.
Many thanks to @spicyj for fixing the issue so quickly.