Elastislide | responsive image carousel that will adapt | Animation library

It won't work that way, the way you are trying to change it. Above all this repo seems to be abandoned since there are issues from 2015 which are not addressed even once by the repo owner.

Anyway, coming to changing it the way you are trying to do it, you first need to understand what the script is actually doing. If you have to add your custom HTML inside the li and use the slider you need to look into how it assigns the max-width and width, rather than trying to force to override it inside the css.

Looking into jquery.elastislide.js what it is doing is that it assign's the max-width to the items/lis in the _setItemSize() function where it uses imgSize object's property width to assign the max-width css property to the item.