column shortcodes stopped working – no float right

The column shortcodes on this site stopped working a few weeks ago. The [one_half_last] column does not float right, and overwrites the other columns. The full-width content after [/one_half_last] does not clear. The last column shortcodes appear to be missing the float:right in the stylesheet, but I am having difficulty updating the stylesheet or perhaps finding the right one to update.
I did not design or develop this website but I maintain it and keep it updated. The column shortcodes had been in place and working for years. No changes had been made to any of the stylesheets. A WordPress update may have been installed. Also, there may have been an update to the underlying twenty-twelve theme this site is based on, although it has a child theme.
When the shortcodes stopped working we removed them from all pages since the content was unreadable. The test page shows the shortcodes not working, followed by hand-applied inline styles that do work. Any insights are appreciated.

The page I need help with: [log in to see the link]