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Hi @anguria65,
We are sorry but NextGen Gallery strips out the majority of HTML tag attributes ( for the security reasons ) using M_NextGen_Pro_I18N::get_kses_allowed_html() method.
Theoretically, you can substitute the call to M_NextGen_Pro_I18N::get_kses_allowed_html() with something that returns your own list of HTML elements and allowed classes but that’d require coding skills.