do you think you can adapt it to display an entire <span> ? That's very neat, but just single string text is not enough for me!

It's wouldn't be possible if you're only using CSS. See this stackoverflow post [1], or more specifically the spec it references [2] on the CSS content property (used by this library).

The acceptable values are:

    normal | none | [ string | uri | counter | attr(identifier) | open-quote | close-quote | no-open-quote | no-close-quote ]+ | inherit

You might think, maybe you could insert html by putting the html on an attribute and using the attr() value, but reading further:

This function returns as a string the value of attribute X for the subject of the selector. The string is not parsed by the CSS processor.

[1] http://stackoverflow.com/questions/4505093/css-content-prope...

[2] http://www.w3.org/TR/CSS21/generate.html#propdef-content

too bad ... I guess it means we're stuck with JS to display multiline and/or format tooltips ...

for the time being, yes.

You mean you want to show HTML inside the tooltip?

yes indeed, especially to allow multiline. (but maybe that's not your original intend ... ?)