Simple CSS Tooltips are … simple CSS only tooltips.
Single line tooltip:
<img src="path/to/foo.png" data-tt="Tooltip text" />
Contents of @data-tt preserve newlines, like pre, and can be used to
produce multiline tooltips.
If you need to position the tooltip somewhere other than below (south of) the
element, use one of the classes .tt-e, .tt-w, .tt-n which
correspond to east, west and north of the element respectively.