Actually, it's rather heavily dependent on Javascript as that's how most of the functionality is handled. Plus some server side is likely required to insert the raw text to be searched in the data attributes. I guess you could hand-code all that text inside numerous data attributes but different people may do things differently.

Although, it is clever but rather limited.

... what do you think is handling the key pressing?... It's not just CSS. The only thing this does that's different than iterating over every element is it splits the hiding to the CSS engine.