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Because it breaks the paradigm of

HTML = Content

CSS = Style / Design

JS = Interaction

Once you start to mix them up you start down a hair path

Lightbox functionality encompasses the JS domain (since user is interacting with the page) as well as CSS domain (since the change that is happening on the page is purely of style/design).

So, the breakup which you have given does not unequivocally argue for the use of JS.

It argues more for JS than CSS.

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