A great CSS work! Congrats for the idea and implementation.

Some ideas for user-experience improvement:

- I subscribe to xparadigm's comment: adjust the mouse pointer on the complete cells to get the right pane. I did not notice it before reading the comment.

- Maybe add a red cross in the pane to close it. I did not see an obvious way how to close it

- Another idea : when the pane shows up, the Mendeleev table could resize down to keep being fully visible.

Already great as it is.

FYI: I browsed your page with IE11 :-/

Looks great! I would alter the look slightly to ensure the atomic numbers are closest to the symbols they refer to. At the moment, your atomic numbers for one line seem to be closer to the symbols of the next line; this might be confusing at a glance.

Good job. Maybe if you changed the cursor property to pointer in search results. Also in table, not only over atomic number but also over the chemical symbol, maybe.