That depends... if it's part of a 3rd party theme or plugin, it's probably an error in the configuration or a bug in the theme/plugin itself. If it's part of a custom (or child) theme, the code would be somewhere inside
wp-content/themes/name-of-theme. Now there's a
no-js class on the
html element, which suggests the desired script isn't even getting loaded...
As for the slider, that's definitively a plugin (you can find it in
wp-content/plugins/revslider), so I think it's just not getting initialised properly. You're generally not supposed to modify plugin code, as it would be getting overwritten with the next update.