I think it's preferable to call this class something like "tabs--active-tab" instead of "is-active" for two reasons:

1. It reduces the likelihood of conflict with another component. If there's another component that uses an "is-active" class for a different purpose the class could be added/removed from the element unexpectedly.

2. It makes it easier to debug the code (especially on production when all the CSS has been minified into an single file). Someone inspecting the DOM can see the class on the element and confidently know that it was added by the "tabs" component.