Is using camelCase in CSS ids or classes ok or not?

Technically, no, there are no technical issues. Do what you like.

Do try to follow a good style-guide though, like this one.

There is one technical limitation if you use camelCase identifiers in your CSS - the |= selector specifier:

<form class="registration"></form>
<form class="search-movies"></form>
<form class="search-actress"></form>

To match only search forms, you can write:

[class|="search"] { font-size: 150% }

You cannot do this with camelCase class names.

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