Hi Philippe, First I think we would benefit from greater detail around what your customer is trying to accomplish. That being said...
I tested the scenario above in the following way:
1) Set a split segmentation with a scope of Pre-Loader: 50% to A and 50% to B in a cookie called 'css_split_test'
2) Added a content modification extension that added a style with the condition that my cookie from the split segmentation cookie has a value of "A"
3) Duplicated the original content modification extension changed the style slightly and changed the condition to value "B"
This worked in my test environment, but that doesn't mean it's necessarily the best solution.
From a troubleshooting standpoint here are the questions that I would start with:
0) are there any JS errors shown in the browser JS console? If so, are they interrupting utag.js?
1) Do all of the segments being set by the Split Segment extension add up to 100%? (If they do not, and are either under OR over 100% unexpected behavior will ensue)
2) Is the proper identifier being used in the content modification extensions?
3) Is the correct class being defined for the content modification (if it's defining a new class that isn't called by the desired objects it won't work or if the existing class is improperly identified)
4) Is the split segment cookie getting set before the content modification extensions run?
4a) if you disable the split segment extension and then manually set the cookie via browser developer tools and refresh the page -- what is the behavior?
If the cookie is not getting set before your content modification extensions run their conditions will always evaluate to false
5) What is the condition you are using for the content modification extensions?
5a) is it evaluating in the expected manner?